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

[Krzysztof Parzyszek via cfe-commits]

kparzysz wrote:

> In general, we aim to only depend on libClangDriver and move it to LLVM so that Flang need not depend on Clang or its libraries. If this patch increases the dependency on Clang it might not be acceptable.

Agreed. 👍

@JDPailleux:  It may a good idea to extract the diagnostic engine into a separate (from clang) library that could be built on its own.  Still, this would require a discussion on Discourse before changing anything.

Flang already handles diagnostic messages on its own, so using clang's diagnostic engine it would have to offer clear advantages.  If you want to start a conversation, please think of those.

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