Dejargonize `subst`

In favor of rust-lang#110793, replace almost every occurence of `subst` and `substitution` from rustc codes, but they still remains in subtrees under `src/tools/` like clippy and test codes (I'd like to replace them after this)

Rollup merge of rust-lang#120958 - ShoyuVanilla:remove-subst, r=oli-obk

Dejargonize `subst`

In favor of rust-lang#110793, replace almost every occurence of `subst` and `substitution` from rustc codes, but they still remains in subtrees under `src/tools/` like clippy and test codes (I'd like to replace them after this)

…ram, r=notriddle

rustdoc: replace `clean::InstantiationParam` with `clean::GenericArg`

Probably better known as `SubstParam` (until rust-lang#120958 which should've probably renamed it to `InstantiatedParam` but anyways).

It doesn't make any sense to me why it should exist as a separate type. `GenericArg` is exactly what we want here anyways from a semantic perspective. Both have the same size btw.

I also took the liberty of doing some drive-by cleanups.

…ram, r=notriddle

rustdoc: replace `clean::InstantiationParam` with `clean::GenericArg`

Probably better known as `SubstParam` (until rust-lang#120958 which should've probably renamed it to `InstantiatedParam` but anyways).

It doesn't make any sense to me why it should exist as a separate type. `GenericArg` is exactly what we want here anyways from a semantic perspective. Both have the same size btw.

I also took the liberty of doing some drive-by cleanups.

Rollup merge of rust-lang#120999 - fmease:rustdoc-rm-instantiation-param, r=notriddle

rustdoc: replace `clean::InstantiationParam` with `clean::GenericArg`

Probably better known as `SubstParam` (until rust-lang#120958 which should've probably renamed it to `InstantiatedParam` but anyways).

It doesn't make any sense to me why it should exist as a separate type. `GenericArg` is exactly what we want here anyways from a semantic perspective. Both have the same size btw.

I also took the liberty of doing some drive-by cleanups.