[clang] [Clang][CodeGen] Fix __builtin_counted_by_ref for nested struct FAMs (#182575) (PR #182590)

[Kees Cook via cfe-commits]

kees wrote:

> Hmm...That might not be enough.

I think it's correct as you have it. See https://llvm.org/docs/GitHub.html#backporting

Milestone tag looks to be sufficient.

https://github.com/llvm/llvm-project/pull/182590