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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> For us, these crashes block a solid piece of internal testing, thus new unnoticed errors accumulate and make things exponentially harder for the future.

If you have a solid piece of internal testing blocked by this, can you provide us with more data? Ideally a working reproducer. This would be more productive than just reverting.

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