Rollup merge of #130788 - tgross35:memchr-pinning, r=Noratrieb,Mark-Simulacrum

Pin memchr to 2.5.0 in the library rather than rustc_ast

The latest versions of `memchr` experience LTO-related issues when compiling for windows-gnu [1], so needs to be pinned. The issue is present in the standard library.

`memchr` has been pinned in `rustc_ast`, but since the workspace was recently split, this pin no longer has any effect on library crates.

Resolve this by adding `memchr` as an _unused_ dependency in `std`, pinned to 2.5. Additionally, remove the pin in `rustc_ast` to allow non-library crates to upgrade to the latest version.

Link: rust-lang/rust#127890 [1]

try-job: x86_64-mingw
try-job: x86_64-msvc

Author: tgross35