[clang] [flang] [flang] Add options -W[no-]unused-dummy-argument and -W[no-]unused-variable (PR #127214)

[Jean-Didier PAILLEUX via cfe-commits]

JDPailleux wrote:

> Thanks for working on this @JDPailleux.
> 
> This looks like a substantial change including to parts of clang. Is it possible to split this into two. One PR for just the "infrastructure" and another for the flang-specifc warnings. This would make it easier to focus the discussion on the design. Could you also provide a high-level description of the approach that you have taken?
> 
> Thanks.

Yes of course, I'll split this in 2 PRs.

https://github.com/llvm/llvm-project/pull/127214