[clang] [libcxx] [clang][ThreadSafety] Handle mutex scope (PR #157171)

[Prabhu Rajasekaran via cfe-commits]

Prabhuk wrote:

> Is there anything in this that requires pch? Seems like the other thread safety tests would make more sense.

Extracted regression tests into clang/test/Analysis directory into a separate file. PTAL.

https://github.com/llvm/llvm-project/pull/157171