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

[via cfe-commits]

bgra8 wrote:

> I hope you realize this is self-imposed limitation that Clang community doesn't necessary have to respect.

Well, the actual LLVM policy is to revert to green: https://llvm.org/docs/DeveloperPolicy.html#patch-reversion-policy

```
When should you revert your own change?

* Any time you learn of a serious problem with a change, you should revert it. We strongly encourage “revert to green” as opposed to “fixing forward”. We encourage reverting first, investigating offline, and then reapplying the fixed patch - possibly after another round of review if warranted.
```

This I think qualifies as a serious problem as it introduces a crash which may occur in *any* existent codebase. Anyone using the compiler at trunk and encountering the crash has no way to know how to deal with it.



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