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

[Timm Baeder via cfe-commits]

tbaederr wrote:

I've resorted to do this in `ThreadSafety.cpp` now, since all other places where we merge function definition and declaration happen before we have the late-parsed attributes available.

>From a diagnostic POV, this seems quite complicated since we have N declarations and need to report differences in M attributes (and each one of them can have >=1 parameter...). For the POC implementation here, I've left if with just reporting a mismatch in attributes. For example with `test/SemaCXX/warn-thread-safety-analysis.cpp`:
```console
error: 'expected-warning' diagnostics seen but not expected:
  File /home/tbaeder/code/llvm-project/clang/test/SemaCXX/warn-thread-safety-analysis.cpp Line 2171: attribute mismatch between function definition and declaration of 'foo2'
  File /home/tbaeder/code/llvm-project/clang/test/SemaCXX/warn-thread-safety-analysis.cpp Line 2172: attribute mismatch between function definition and declaration of 'foo3'
  File /home/tbaeder/code/llvm-project/clang/test/SemaCXX/warn-thread-safety-analysis.cpp Line 2222: attribute mismatch between function definition and declaration of 'fooF2'
  File /home/tbaeder/code/llvm-project/clang/test/SemaCXX/warn-thread-safety-analysis.cpp Line 3182: attribute mismatch between function definition and declaration of 'foo1'
  File /home/tbaeder/code/llvm-project/clang/test/SemaCXX/warn-thread-safety-analysis.cpp Line 3183: attribute mismatch between function definition and declaration of 'foo2'
  File /home/tbaeder/code/llvm-project/clang/test/SemaCXX/warn-thread-safety-analysis.cpp Line 5220: attribute mismatch between function definition and declaration of 'test'
  File /home/tbaeder/code/llvm-project/clang/test/SemaCXX/warn-thread-safety-analysis.cpp Line 5221: attribute mismatch between function definition and declaration of 'test2'

```

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