[clang-tools-extra] Add new tool: clang-read-diagnostics (PR #118522)

[Yuxuan Chen via cfe-commits]

yuxuanchen1997 wrote:

> > Looking at the test failure, it appears that when only building `check-clang-tools`, no `clang` binary was built. So the test driver cannot find `clang`. Maybe this diagnostics binary file should be checked in for the test? That will not catch future errors if the diagnostics binary format changes.
> 
> We should stay away from committed binaries, per the `xz` fiasco. A better way to solve this, I think, would be to add a `REQUIRES:` clause for it. See the `available_features.add()` calls in `llvm/utils/lit/lit/llvm/config.py`.

Great. This should work now. 

https://github.com/llvm/llvm-project/pull/118522