Relink don't rebuild: add a baseline, sound implementation that can be incrementally improved

[susitsm]

This PR implemenets a sound but not too useful implementation of relink don't rebuild with the unstable -Z public-api-hash flag. It currently uses the stable hash of all items in the metadata.

The goal is to give a base implementation that can be used for experimentation. It can be incrementally improved over time by removing or stable sorting items. The PR also adds new test attributes rustc_public_hash_changed and rustc_public_hash_unchanged and an example test using them.

What are non-goals for this PR: a useful, optimized implementation of RDR and public api hashing. That has many more challenges which will each require careful review.

A non exhaustive list of the challenges for the feature I ran into while trying to make the hash useful (this should probably go in a tracking issue, but I'm not aware of one)

• Name resolution diagnostics use private items. Importing the private my_crate::func prints private function as the error
• Stripped cfg items are included in rmeta for improved diagnostics.
• Which types to include? A simple is this public is not enough. RPIT returned types and the private types used in the included mir for generic/inline functions are probably needed.
• It might make sense to have multiple hashes. Rmeta generation (cargo check), could use a much simpler hash than codegen, it does not need private types from mir (or any mir at all?)

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

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

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @jdonszelmann

rustbot has assigned @jdonszelmann.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 20 candidates