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

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cor3ntin wrote:

> Don't think I can get the Windows+libcxx pre-commit CI working :|
> 
> `libcxx/gdb/gdb_pretty_printer_test.sh.cpp` has `UNSUPPORTED` lines for clang 16-18 but 19 is missing now. Not sure how this works for others.

We have this issue on other PRs, I think we can ignore. @ldionne 

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