Move feature* methods from parse mod to errors mod.

[nnethercote]

As the FIXME comment says, these no longer use ParseSess and so the parse mod is not a good place for them. The errors mod is a better home.

r? @TaKO8Ki

[rustbot]

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

HIR ty lowering was modified

cc @fmease

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to constck

cc @fee1-dead

changes to the core type system

cc @lcnr

[TaKO8Ki]

@nnethercote This is not blocking, but rustc-dev-guide still mentions rustc_session::parse::feature_err.

https://github.com/rust-lang/rustc-dev-guide/blob/b1026059fb05aa7bd2d9380d444ff864b3a449cc/src/implementing-new-features.md?plain=1#L194

[TaKO8Ki]

@bors r+ rollup

[rust-bors]

📌 Commit 6d79bc6 has been approved by TaKO8Ki

It is now in the queue for this repository.