[clang] [Clang][Parse] Fix crash on stmt-expr ending with 'extern void;' (PR #181579)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Co-pilot isn't relevant here, but can you clarify what portions of this PR were authored by you, and what by AI, in accordance with llvm policy?

https://github.com/llvm/llvm-project/pull/181579