[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?
> 
> 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).

I'd be more than happy to drop this patch if the alternative is better. I believe that we shouldn't need a tool like this, as the majority of third-party tooling won't want to implement reading from a proprietary format. It adds moving parts while getting little in return (maybe encoding/decoding speed or file size).

Things could also be in a friendlier format than XML. We currently use YAML for middle end opt remarks but that's different from diagnostics engine. 

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