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

[Aaron Ballman via cfe-commits]

AaronBallman 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?

I don't think either is preferred over the other, but I was unaware that we had something which logs to XML, so it's possible there's bitrot there (it seems we may not have any direct test coverage of that feature, at least from my cursory look).

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