[clang] [Clang] Improve error for `-fsanitize=function/kcfi -mexecute-only` incompatibility (PR #118816)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

Yes, err_drv_unsupported_opt_for_target will be better for the incompatible triple error

https://github.com/llvm/llvm-project/pull/118816