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

[Timm Baeder via cfe-commits]

tbaederr wrote:

I will also merge https://github.com/llvm/llvm-project/pull/80023 once the CI is green. That will fix one potential source of problems.

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