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

[Csanád Hajdú via cfe-commits]

Il-Capitano wrote:

Thanks for reviewing!

I updated the code to use `err_drv_unsupported_opt_for_target`, and added test cases for the `-mpure-code` alias of `-mexecute-only` also.

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