[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. 

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