[clang] [Sema] Fix crash on function declaration inside nested expansion statements (PR #211649)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Made https://github.com/llvm/llvm-project/pull/211745 which I think is plausibly addressing the root cause, though no guarantee of correctness.

https://github.com/llvm/llvm-project/pull/211649