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

[David Truby via cfe-commits]

DavidTruby wrote:

Some extra context: there was a discussion quite some time ago about moving lots of the driver/diagnostic code from `clang` to a separate top-level LLVM project, and I believe it was generally agreed that this would be a good idea: https://discourse.llvm.org/t/rfc-refactor-clang-move-frontend-driver-diagnostics-code-to-llvm/55498/8

As I recall; @banach-space and I (separately) both had a go at doing this and it proved to be quite complex, so we ran out of time. It's still something I'd like to see one day though!

I think until such a thing is done we wouldn't want to couple even more things in `clang/` into flang though, so I'd be concerned about doing this for now.

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