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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I poked the bot to create the pull request for you.  Please leave a comment on #182606 with a risk/benefit analysis for cherry-picking this to the branch.

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