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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@eaeltsin  I'm against reverting this or introducing a switch until we get a reproducer (or author figures out what's wrong). I believe it's a very worthwhile feature, and we want some actionable feedback to improve it. Author has been very responsive so far, so we are good on this side, too.

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