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

[Tom Eccles via cfe-commits]

tblah 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

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