[clang] [Clang-CL][DXC] Expose -fdiagnostic-color= to clang-cl and clang-dxc (PR #120644)

[Mészáros Gergely via cfe-commits]

https://github.com/Maetveis requested changes to this pull request.

Thank you, mostly looks good, I think the tests can be made simpler. And I think adding release notes would benefit users.
You can add a new point to "New Compiler Flags" in the clang release notes that clang-cl and clang-dxc now also accept`-fdiagnostics-color=[auto|never|always]` in addition to `-f[no-]color-diagnostics`.

https://github.com/llvm/llvm-project/pull/120644