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

[Timm Baeder via cfe-commits]

tbaederr wrote:

> 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.

Of course, but a color theme should make sure that blue text is readable.

We have a _very_ limited set of colors available in `llvm::raw_ostream`. I could imagine yellow working better, but I have no idea if yellow on white will be significantly better to read on compiler-explorer.

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