[clang] [clang][Diagnostics] Highlight code snippets (PR #66514)

[via cfe-commits]

bgra8 wrote:

> As mentioned above, `-fno-color-diagnostics` is the flag you can deploy internally this very moment.

Disabling color diagnostics would make the job of other software engineers harder, right? This hammer is a bit too big for the problem at hand, right?

https://github.com/llvm/llvm-project/pull/66514