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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman
> 
> > Do you see this crash with community clang and no downstream changes?
> 
> Yes I can reproduce it with a vanila `clang` built from a git checkout at this version, but the problem is extremely sensitive to the input headers, their content and defines (`-Dbla`) in the compilation command.

Thank you, that's good to know you hit it with vanilla Clang! I think that between this information and the backtrace in #80127 we have enough of a reason to revert without a reproducer at hand. However, I think that if we don't have a reproducer by mid-next week (say 22:00 UTC on 2/7, but the date/time are negotiable) we should re-land the changes as-is to try to get more instances of the crash happening in the wild to help us solve the issue. WDYT?

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