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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@bgra8 Thank you! This is helpful.

> The problem is extremely hard to reduce as it happens during preprocessing and even removing unrelated headers causes the crash to disappear.

Yeah, crashes happening at this stage can easily be insane to reduce manually.

> This crash is blocking our internal work at Google. I second @eaeltsin's proposal to put the highlightLines() function behind a flag at least until we can get to the bottom of this.

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

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