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

[Timm Baeder via cfe-commits]

tbaederr wrote:

Here's gnome-terminal's default colors, first normal and then bright:
![Screenshot from 2024-01-30 14-55-37](https://github.com/llvm/llvm-project/assets/49720664/9f225494-0cc9-4d53-a0e1-98b45b5b4c4b)

Here the same with xterm:
![Screenshot from 2024-01-30 14-58-24](https://github.com/llvm/llvm-project/assets/49720664/b7e03a49-eacd-4fc6-bb30-b703df9dfceb)


This is Konsole:
![Screenshot from 2024-01-30 15-01-06](https://github.com/llvm/llvm-project/assets/49720664/988e9a64-5899-4f23-baaf-c7961766ad4f)
 (I checked in an image editor and the blue used for `static_cast` is indeed every so slightly different)

And here is Konsole with its "Black on light yellow" color scheme:
![Screenshot from 2024-01-30 15-05-16](https://github.com/llvm/llvm-project/assets/49720664/8ee801e0-5ce6-4047-8ca3-b4158d2a3068)





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