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

[via cfe-commits]

NexMing wrote:

> I tend to prefer @jeanPerier's suggestion of having the option name reflect that only the LLVM dialect is being used.
> 
> In the future, do you intend to provide a way to choose the set of dialects to use? In that case, we could consider something a bit more general like @tblah's suggestion.

Currently, using the LLVM dialect is only a temporary solution and doesn’t align with my long-term goals. In the future, I plan to implement conversions from FIR to scf, memref, and other core dialects. I personally prefer @tblah’s suggestion, but as @jeanPerier pointed out, I’m indeed not yet sure how to make a difference between the different levels of representation that the core dialects. I think the appropriate core MLIR dialects are those that most closely reflect the original FIR representation. Do you have any suggestions on how to approach this?

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