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

[Timm Baeder via cfe-commits]

tbaederr wrote:

Okay, that seems quite complex since a lexer with a preprocessor doesn't seem to return comments at all and the tokens I get are all identifiers and don't have their `kw_` state set. Provided what I'm doing isn't completely wrong of course.

I'm open to suggestions of course, I don't use the lexer and preprocessor API very often.

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