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

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

> > Doesn't `counted_by` have the same requirements? If not, why does guarded_by?
> 
> It does, AFAIK.

But counted_by _isn't_ using `-fexperimental-late-parse-attributes` IIUC. Do they have the same problem then of needing to reorder member declarations? cc @kees @isanbard @bwendling 

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