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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for the fix, this looks like it's heading in the right direction! One additional test case to add just to be sure it works as expected, but otherwise LG. Please also add a release note to `clang/docs/ReleaseNotes.rst` so users know about the fix.

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