[clang] [Clang] Fix crash for _Countof(void) by correcting extension handling (PR #181169)

[Aadarsh Keshri via cfe-commits]

Aadarsh-Keshri wrote:

@AaronBallman sir I did the suggested test, added it and updated `clang/docs/ReleaseNotes.rst`.

https://github.com/llvm/llvm-project/pull/181169