Exposing `-fdiagnostic-color=` to clang-cl and clang-dxc.
`-fcolor-diagnostics` and `-fno-color-diagnostics` are already allowed
in both of these and `-fdiagnostics-color=` allows one additional value,
`auto`.
I've added the tests for clang-cl to `cl-options.c` as per the comments
in the issue linked below. I couldn't finding a suitable existing file
to add the clang-dxc tests to so I've created a new one.
Resolves#119184
When these options were enabled for flang the visibility was also
extended to clang-cl and dxc. This was due to a
misunderstanding of the default value for `Visibility`.
