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

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> A new parent class for code-gen frontend actions is introduced:`CodeGenAction`.

Should this instead be a "A new code-gen action class is introduced: `EmitMLIRAction`?" That seems to be the only new class that has been introduced and it is not (yet, at least) a parent for any other class.

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