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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Ah, I see where the limitation comes from.
According to [Wikipedia](https://en.wikipedia.org/wiki/ANSI_escape_code#3-bit_and_4-bit), there is bright blue color which many terminals display differently from regular blue. While this means we're expanding 3-bit color palette to 4-bit color palette, I'm not sure how much of a compatibility issue this is in practice.

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