[clang] [flang] [flang][driver] Separate the actions of the `-emit-fir` and `-emit-mlir` options (PR #139857)

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NexMing wrote:

> > > Who said it is a temporary solution? Can you point to an RFC?
> > 
> > 
> > This specific patch wasn't discussed in the RFC but there is some discussion here https://discourse.llvm.org/t/rfc-add-fir-affine-optimization-fir-pass-pipeline/86190/5
> 
> So is that only for the affine pipeline?

It's not just about Affine. In the future, I plan to implement conversions from FIR to scf, memref, and other core dialects. The FIR → Affine → FIR path is part of my experimental roadmap for exploring Fortran optimizations. 
 My envisioned final pipeline is: FIR → core MLIR (do optimization. ,like SCF->Affine )→ LLVM.
I will revise the RFC title or create a new RFC.

https://github.com/llvm/llvm-project/pull/139857