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#127890 [1]

Author: tgross35

Cargo.lock

@@ -23,6 +23,10 @@ unwind = { path = "../unwind" }
 hashbrown = { version = "0.14", default-features = false, features = [
     'rustc-dep-of-std',
 ] }
+# FIXME(#127890): `object` depends on `memchr`, but `memchr` > v2.5 causes
+# issues with LTO. This dependency is not used directly, but pin it here so
+# it resolves to 2.5. To be removed once rust-lang/rust#127890 is fixed.
+memchr = { version = "=2.5.0", default-features = false, features = ["rustc-dep-of-std"] }
 std_detect = { path = "../stdarch/crates/std_detect", default-features = false, features = [
     'rustc-dep-of-std',
 ] }