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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

It looks a tinybit better in my local terminal, but still significantly harder to read than the rest of the text:
![image](https://github.com/llvm/llvm-project/assets/12883766/5b0136a5-012b-47f5-b888-787d1c060d54)

But it's a bit off the point. We should pick colors that have good contrast against both light and dark color themes by default, and not ask everyone to change their color palette. Also make sure to zoom out when you look at this yourself: issue is more apparent this way.

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