Flags specifics to a language mode are not diagnosed in other language modes

[cor3ntin]

clang -x c -fexceptions -frtti -fchar8_t -fcoroutines -fcxx-modules

We do not emit diagnostics for -fexceptions -frtti -fchar8_t, which are not valid C options.

(but we seem to do for -fcoroutines -fcxx-modules)

https://compiler-explorer.com/z/48hc6Krh3

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: cor3ntin (cor3ntin)

``` clang -x c -fexceptions -frtti -fchar8_t -fcoroutines -fcxx-modules ```

We do not emit diagnostics for -fexceptions -frtti -fchar8_t, which are not valid C options.

(but we seem to do for -fcoroutines -fcxx-modules)

https://compiler-explorer.com/z/48hc6Krh3