[llvm] [LLVMgold] Suppress -Wcast-function-type-mismatch diagnostic (PR #89994)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I think we could use a union, but the mechanism would be too heavy.

> [llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library](https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library) lists Clang 5.0, we are using clang 12.

While we support older clang versions down to 5, we cannot support `-Werror` for all of them, since every version might fix some false positives present in older versions...


https://github.com/llvm/llvm-project/pull/89994