…=Kobzol

build `libcxx-version` only when it doesn't exist

In rust-lang#126423, it seems like c++ parsing takes quite amount of time on bootstrap startups. This PR makes libcxx-version to be compiled only when it doesn't exist.

A simple demonstration on the overhead of buiding `libcxx-version`:

```sh
$ rm -rf build/host/libcxx-version

$ x build
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.07s

 ----- LIBCXX VERSION CHECK TOOK: 509ms
Building tool rustdoc (stage1 -> stage2, x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.25s
Build completed successfully in 0:00:02

$ x build
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.07s

 ----- LIBCXX VERSION CHECK TOOK: 2ms
Creating a sysroot for stage2 compiler (use `rustup toolchain link 'name' build/host/stage2`)
Building tool rustdoc (stage1 -> stage2, x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.14s
Build completed successfully in 0:00:01
```

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#125722 (Indicate in `non_local_defs` lint that the macro needs to change)
 - rust-lang#125829 (rustc_span: Add conveniences for working with span formats)
 - rust-lang#126128 (Consistently use subtyping in method resolution)
 - rust-lang#126192 (Various Redox OS fixes and add i686 Redox OS target)
 - rust-lang#126352 (ci: Update centos:7 to use vault repos)
 - rust-lang#126354 (Use `Variance` glob imported variants everywhere)
 - rust-lang#126469 (MIR Shl/Shr: the offset can be computed with rem_euclid)
 - rust-lang#126472 (build `libcxx-version` only when it doesn't exist)

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

…=Kobzol

build `libcxx-version` only when it doesn't exist

In rust-lang#126423, it seems like c++ parsing takes quite amount of time on bootstrap startups. This PR makes libcxx-version to be compiled only when it doesn't exist.

A simple demonstration on the overhead of buiding `libcxx-version`:

```sh
$ rm -rf build/host/libcxx-version

$ x build
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.07s

 ----- LIBCXX VERSION CHECK TOOK: 509ms
Building tool rustdoc (stage1 -> stage2, x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.25s
Build completed successfully in 0:00:02

$ x build
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.07s

 ----- LIBCXX VERSION CHECK TOOK: 2ms
Creating a sysroot for stage2 compiler (use `rustup toolchain link 'name' build/host/stage2`)
Building tool rustdoc (stage1 -> stage2, x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.14s
Build completed successfully in 0:00:01
```

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#125722 (Indicate in `non_local_defs` lint that the macro needs to change)
 - rust-lang#125829 (rustc_span: Add conveniences for working with span formats)
 - rust-lang#126192 (Various Redox OS fixes and add i686 Redox OS target)
 - rust-lang#126352 (ci: Update centos:7 to use vault repos)
 - rust-lang#126354 (Use `Variance` glob imported variants everywhere)
 - rust-lang#126469 (MIR Shl/Shr: the offset can be computed with rem_euclid)
 - rust-lang#126472 (build `libcxx-version` only when it doesn't exist)
 - rust-lang#126476 (Fix running bootstrap tests with a local Rust toolchain as the stage0)

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

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#125722 (Indicate in `non_local_defs` lint that the macro needs to change)
 - rust-lang#125829 (rustc_span: Add conveniences for working with span formats)
 - rust-lang#126192 (Various Redox OS fixes and add i686 Redox OS target)
 - rust-lang#126352 (ci: Update centos:7 to use vault repos)
 - rust-lang#126354 (Use `Variance` glob imported variants everywhere)
 - rust-lang#126469 (MIR Shl/Shr: the offset can be computed with rem_euclid)
 - rust-lang#126472 (build `libcxx-version` only when it doesn't exist)
 - rust-lang#126476 (Fix running bootstrap tests with a local Rust toolchain as the stage0)

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

…=Kobzol

build `libcxx-version` only when it doesn't exist

In rust-lang#126423, it seems like c++ parsing takes quite amount of time on bootstrap startups. This PR makes libcxx-version to be compiled only when it doesn't exist.

A simple demonstration on the overhead of buiding `libcxx-version`:

```sh
$ rm -rf build/host/libcxx-version

$ x build
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.07s

 ----- LIBCXX VERSION CHECK TOOK: 509ms
Building tool rustdoc (stage1 -> stage2, x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.25s
Build completed successfully in 0:00:02

$ x build
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.07s

 ----- LIBCXX VERSION CHECK TOOK: 2ms
Creating a sysroot for stage2 compiler (use `rustup toolchain link 'name' build/host/stage2`)
Building tool rustdoc (stage1 -> stage2, x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.14s
Build completed successfully in 0:00:01
```

…iaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#125722 (Indicate in `non_local_defs` lint that the macro needs to change)
 - rust-lang#125829 (rustc_span: Add conveniences for working with span formats)
 - rust-lang#126192 (Various Redox OS fixes and add i686 Redox OS target)
 - rust-lang#126352 (ci: Update centos:7 to use vault repos)
 - rust-lang#126354 (Use `Variance` glob imported variants everywhere)
 - rust-lang#126469 (MIR Shl/Shr: the offset can be computed with rem_euclid)
 - rust-lang#126472 (build `libcxx-version` only when it doesn't exist)

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

…iaskrgr

Rollup of 9 pull requests

Successful merges:

 - rust-lang#126354 (Use `Variance` glob imported variants everywhere)
 - rust-lang#126367 (Point out failing never obligation for `DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK`)
 - rust-lang#126469 (MIR Shl/Shr: the offset can be computed with rem_euclid)
 - rust-lang#126471 (Use a consistent way to filter out bounds instead of splitting it into three places)
 - rust-lang#126472 (build `libcxx-version` only when it doesn't exist)
 - rust-lang#126497 (delegation: Fix hygiene for `self`)
 - rust-lang#126501 (make bors ignore comments in PR template)
 - rust-lang#126509 (std: suggest OnceLock over Once)
 - rust-lang#126512 (Miri subtree update)

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