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

[Marco Elver via cfe-commits]

melver wrote:

Thanks for the feedback. Addressed comments as best as I could.

Most notable changes:
- Also warns properly for loops with mismatching reentrancy depth.
- Devirtualized new helpers.
- Require ordering `reentrant_capability` after `capability`.
- Stylistic improvements.

PTAL.

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