[clang] Thread Safety Analysis: Support warning on taking address of guarded variables (PR #123063)

[Marco Elver via cfe-commits]

melver wrote:

 [...]
> Then we only need to make sure that `checkPtAccess` can look through `&`, as mentioned above. (Casts should already be unwrapped.) This might not even need a new flag, it's just closing a gap in the existing analysis.

Thanks for the suggestions!

I implemented both -- please see https://github.com/llvm/llvm-project/pull/127396.

https://github.com/llvm/llvm-project/pull/123063