Warn on codegen attributes on required trait methods

[JonathanBrouwer]

This PR turns applying the following attributes on required trait methods (that is, trait methods without a default implementation) into a FCW:

• #[cold]
• #[link_section]
• #[linkage] (unstable)
• #[rustc_allow_const_fn_unstable] (internal attribute)

These attributes already had no effect when applied to a required trait method, this PR only adds a warning.

Furthermore, it adds a comment in the code that the following codegen attributes are inherited when applied to a required trait method:

• #[track_caller]
• #[align] (unstable)

@rustbot labels +I-lang-nominated
@rust-lang/lang

Two questions for the lang team:

• Is adding this warning ok?
• Does the current behaviour of these attributes align with that you would expect them to be?

Fixes #147432

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[rustbot]

r? @jackh726

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[jdonszelmann]

r? jdonszelmann

[traviscross]

@rfcbot fcp merge lang

[rust-rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
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