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

[Yuxuan Chen via cfe-commits]

yuxuanchen1997 wrote:

> so we don't need to carry around multiple implementations of machine-readable diagnostic information.

I just realized that we also have `-diagnostic-log-file` as a cc1 flag. This logs into an XML. Is this preferred to the binary format or no? 

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