[clang] [Clang][counted_by] Use the expr's RecordDecl that transitively contains the counter field (PR #205903)

[Bill Wendling via cfe-commits]

isanbard wrote:

Okay. I think this is now ready for review. I changed the code to find the best base RecordDecl to build the GEP off of. PTAL.

https://github.com/llvm/llvm-project/pull/205903