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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

The deep shade of blue doesn't make sense for both light and dark backgrounds:
![image](https://github.com/llvm/llvm-project/assets/12883766/e9ee6658-1ab1-422f-914b-5f95cac3846d)
![image](https://github.com/llvm/llvm-project/assets/12883766/986ae6f4-2fa9-4d2a-b661-47de3bf92cde)

It's too dark on a light background to be easily confused with black, and dark enough on a dark background to easily blend in.

Note that screenshots are intentionally big, so that diagnostics text is small enough to represent how it looks in the context. Isolated screenshots of diagnostic text (like ones posted in this thread earlier) make it too big, alleviating the issue.

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