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

[Timm Baeder via cfe-commits]

tbaederr wrote:

![Screenshot from 2023-09-20 15-26-50](https://github.com/llvm/llvm-project/assets/49720664/763b0f28-3b05-4843-b71b-0465fba110ca)

Slightly sad that `true`/`false`/`nullptr` are also marked as keywords now, I was hoping they would be literals.

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