[clang] [flang] [flang/clang] Adding use of Clang's diagnostics in Flang (PR #130593)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

For compiler options, we have various "categories" in `Options.td`, `ClangOptions`, `FlangOptions` etc. which only apply to specific frontends. I haven't looked very closely at this, but I believe that there is only one "category" for warnings. Is this correct? If that the case, perhaps we could consider if there is a way to change changing the way warnings are handled _in the driver_? If we can split the warnings into "categories", we just need to check for all warnings in category. That may not introduce any additional dependencies on clang.

I hasten to stress that I have not looked into how feasible such an approach is, or if it is even possible. But it may be less effort than completely disentangling the diagnostics code from clang.

https://github.com/llvm/llvm-project/pull/130593