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

[Valentin Clement バレンタイン クレメン via cfe-commits]

clementval wrote:

>Currently, using the LLVM dialect is only a temporary solution and doesn’t align with my long-term goals.

Who said it is a temporary solution? Can you point to an RFC? 

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