expand: Leave traces when expanding `cfg_attr` attributes

Currently `cfg_trace` just disappears during expansion, but after this PR `#[cfg_attr(some tokens)]` will leave a `#[cfg_attr_trace(some tokens)]` attribute instead of itself in AST after expansion (the new attribute is built-in and inert, its inner tokens are the same as in the original attribute).
This trace attribute can then be used by lints or other diagnostics, rust-lang#133823 has some examples.

Tokens in these trace attributes are set to an empty token stream, so the traces are non-existent for proc macros and cannot affect any user-observable behavior.
This is also a weakness, because if a proc macro processes some code with the trace attributes, they will be lost, so the traces are best effort rather than precise.

The idea belongs to `@estebank.`

expand: Leave traces when expanding `cfg_attr` attributes

Currently `cfg_trace` just disappears during expansion, but after this PR `#[cfg_attr(some tokens)]` will leave a `#[cfg_attr_trace(some tokens)]` attribute instead of itself in AST after expansion (the new attribute is built-in and inert, its inner tokens are the same as in the original attribute).
This trace attribute can then be used by lints or other diagnostics, rust-lang#133823 has some examples.

Tokens in these trace attributes are set to an empty token stream, so the traces are non-existent for proc macros and cannot affect any user-observable behavior.
This is also a weakness, because if a proc macro processes some code with the trace attributes, they will be lost, so the traces are best effort rather than precise.

The next step is to do the same thing with `cfg` attributes (`#[cfg(TRUE)]` currently remains in both AST and tokens after expanding, it should be replaced with a trace instead).

The idea belongs to `@estebank.`

expand: Leave traces when expanding `cfg_attr` attributes

Currently `cfg_trace` just disappears during expansion, but after this PR `#[cfg_attr(some tokens)]` will leave a `#[cfg_attr_trace(some tokens)]` attribute instead of itself in AST after expansion (the new attribute is built-in and inert, its inner tokens are the same as in the original attribute).
This trace attribute can then be used by lints or other diagnostics, rust-lang#133823 has some examples.

Tokens in these trace attributes are set to an empty token stream, so the traces are non-existent for proc macros and cannot affect any user-observable behavior.
This is also a weakness, because if a proc macro processes some code with the trace attributes, they will be lost, so the traces are best effort rather than precise.

The next step is to do the same thing with `cfg` attributes (`#[cfg(TRUE)]` currently remains in both AST and tokens after expanding, it should be replaced with a trace instead).

The idea belongs to `@estebank.`

expand: Leave traces when expanding `cfg_attr` attributes

Currently `cfg_trace` just disappears during expansion, but after this PR `#[cfg_attr(some tokens)]` will leave a `#[cfg_attr_trace(some tokens)]` attribute instead of itself in AST after expansion (the new attribute is built-in and inert, its inner tokens are the same as in the original attribute).
This trace attribute can then be used by lints or other diagnostics, rust-lang#133823 has some examples.

Tokens in these trace attributes are set to an empty token stream, so the traces are non-existent for proc macros and cannot affect any user-observable behavior.
This is also a weakness, because if a proc macro processes some code with the trace attributes, they will be lost, so the traces are best effort rather than precise.

The next step is to do the same thing with `cfg` attributes (`#[cfg(TRUE)]` currently remains in both AST and tokens after expanding, it should be replaced with a trace instead).

The idea belongs to `@estebank.`

Use `cfg_attr_trace` in AST with a placeholder attribute for accurate suggestion

In rust-lang#138515, we insert a placeholder attribute so that checks for attributes can still know about the placement of `cfg` attributes. When we suggest removing items with `cfg_attr`s (fix rust-lang#56328) and make them verbose. We tweak the wording of the existing "unused `extern crate`" lint.

```
warning: unused `extern crate`
  --> $DIR/removing-extern-crate.rs:9:1
   |
LL | extern crate removing_extern_crate as foo;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
   |
note: the lint level is defined here
  --> $DIR/removing-extern-crate.rs:6:9
   |
LL | #![warn(rust_2018_idioms)]
   |         ^^^^^^^^^^^^^^^^
   = note: `#[warn(unused_extern_crates)]` implied by `#[warn(rust_2018_idioms)]`
help: remove the unused `extern crate`
   |
LL - #[cfg_attr(test, macro_use)]
LL - extern crate removing_extern_crate as foo;
   |
```

r? `@petrochenkov`

Rollup of 7 pull requests

Successful merges:

 - #125087 (Optimize `Seek::stream_len` impl for `File`)
 - #133823 (Use `cfg_attr_trace` in AST with a placeholder attribute for accurate suggestion)
 - #138285 (Stabilize `repr128`)
 - #139994 (add `CStr::display`)
 - #141477 (Path::with_extension: show that it adds an extension where one did no…)
 - #141533 (clean up old rintf leftovers)
 - #141693 (Subtree update of `rust-analyzer`)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 8 pull requests

Successful merges:

 - #133823 (Use `cfg_attr_trace` in AST with a placeholder attribute for accurate suggestion)
 - #141004 (Report text_direction_codepoint_in_literal when parsing)
 - #141407 (Refactor the two-phase check for impls and impl items)
 - #141430 (remove `visit_clobber` and move `DummyAstNode` to `rustc_expand`)
 - #141507 (atomic_load intrinsic: use const generic parameter for ordering)
 - #141538 (implement `va_arg` for x86_64 systemv)
 - #141669 (float: Replace some approximate assertions with exact)
 - #141747 (rustdoc: display doc(cfg(false)) properly)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of #133823 - estebank:issue-56328, r=petrochenkov

Use `cfg_attr_trace` in AST with a placeholder attribute for accurate suggestion

In #138515, we insert a placeholder attribute so that checks for attributes can still know about the placement of `cfg` attributes. When we suggest removing items with `cfg_attr`s (fix #56328) and make them verbose. We tweak the wording of the existing "unused `extern crate`" lint.

```
warning: unused `extern crate`
  --> $DIR/removing-extern-crate.rs:9:1
   |
LL | extern crate removing_extern_crate as foo;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
   |
note: the lint level is defined here
  --> $DIR/removing-extern-crate.rs:6:9
   |
LL | #![warn(rust_2018_idioms)]
   |         ^^^^^^^^^^^^^^^^
   = note: `#[warn(unused_extern_crates)]` implied by `#[warn(rust_2018_idioms)]`
help: remove the unused `extern crate`
   |
LL - #[cfg_attr(test, macro_use)]
LL - extern crate removing_extern_crate as foo;
   |
```

r? `@petrochenkov`

try-job: x86_64-gnu-aux