Auto merge of #154924 - nnethercote:rm-impl-HashStable-for-Attr-slice, r=JonathanBrouwer

Remove `HashStable` impl for `[hir::Attribute]`.



This impl skips:
- All doc comments
- A handful of other attributes, mostly `rustc_*` ones related to incremental compilation testing.
    
This skipping originated in rust-lang/rust#36025 and was extended a couple of times, e.g. in rust-lang/rust#36370. Those PRs don't have any explanation of why the skipping exists. Perhaps the reasoning was that doc comments should only affect rustdoc and rustdoc doesn't use incremental compilation? But doc comments end up in metadata, and there is a query `attrs_for_def` that returns a `&'tcx [hir::Attribute]`. So skipping some attributes just seems plainly wrong.
    
This commit removes the impl, which means `[hir::Attribute]` hashing falls back to the default impl for `[T]`. This has no noticeable effect on the test suite. It does slightly hurt performance, because of the doc comments. This perf regression seems worth it for the correctness benefits.

Author: bors