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

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu commented:

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.

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