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

[Bill Wendling via cfe-commits]

bwendling wrote:

Our current `counted_by` implementation is for flexible array members only, which are always at the end of the struct (or supposed to be). When we start applying them to pointers in structs (coming soon), we'll need to use that flag. However, I think that Apple is __finally__ starting to release some of their features, and I hope that the flag will go away as it'll always be on.

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