[clang] [Clang][Sema] Allow counted_by on void* as GNU extension (PR #164737)

[Kees Cook via cfe-commits]

kees wrote:

FYI, GCC has landed their corresponding change to support `void *` with `counted_by`: https://github.com/gcc-mirror/gcc/commit/6951e953e01a91333bacecb9609ca12f7d83af0a

https://github.com/llvm/llvm-project/pull/164737