[clang] Thread Safety Analysis: Support reentrant capabilities (PR #137133)

[Aaron Puchert via cfe-commits]

https://github.com/aaronpuchert commented:

Two more things come to mind:
* Consider adding a check to `SemaDeclAttr.cpp` that the new attribute is always accompanied by `capability` or `lockable`. Although I wonder whether that's too early. I'm not sure if we can see the other attributes already.
* You should probably do something in `ThreadSafetyAnalyzer::intersectAndWarn`: if two branches join with different counts, we should warn and drop the fact with higher count.

https://github.com/llvm/llvm-project/pull/137133