[clang] [clang] Ignore GCC 11 `[[malloc(x)]]` attribute (PR #68059)

[Erich Keane via cfe-commits]

erichkeane wrote:

also-also: A bug report to improve the diagnostic for `-Wmismatched-dealloc` based on this being in the AST would be appreciated.  If you could file one, it would be greatly appreciated.

Again, sorry for the delay on this one, we managed to drop the ball on this review.

https://github.com/llvm/llvm-project/pull/68059