[clang] [clang] Warn on mismatched RequiresCapability attributes (PR #67520)

[Aaron Puchert via cfe-commits]

https://github.com/aaronpuchert commented:

I've thought a bit more about what inheritance means for this and I think it makes our job easier. We can probably collect capabilities on the first declaration and then just check that the non-inherited attributes on subsequent declarations are already contained in that set.

Also, this should be extended to almost all other thread safety attributes. For a start, do one or two so that we can see what it looks like.

https://github.com/llvm/llvm-project/pull/67520