[clang] [clang] Implement function pointer signing and authenticated function calls (PR #93906)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

Do we have to fix this just to appease gcc 9.2? It looks like newer versions of gcc (after 9.3) don't emit the warning.

https://github.com/llvm/llvm-project/pull/93906