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

[Jon Roelofs via cfe-commits]

jroelofs 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`.

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