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

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cor3ntin wrote:

This is neat!
My biggest concerns is the handling of the keywords. It's very inaccurate and would hard to keep in sync.

I think we could get `TextDiagnostic` to be constructed with a `Preprocessor` object (by modifying the various `BeginSourceFile` implementations, I think)

This would let us call `getIdentifierInfo` on the preprocessor and from then we can determine if a particular identifier is a keyword. Is that something you considered?






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