[CIR] Add option to emit MLIR in LLVM dialect.

[Jezurko]

This PR adds the flag -emit-mlir-llvm to allow emitting of MLIR in the LLVM dialect (cc @xlauko who asked me to do this).
I'm not sure if the naming of the flag is the best and maybe someone will have a better idea.
Another solution would be to make the -emit-mlir flag have a value, that specifies the target dialect (CIR/MLIR std dialects/LLVM Dialect).

[xlauko]

Instead of the current approach, I recommend using -emit-mlir=std to match the behavior of -emit-mlir, which is ambiguous regarding which MLIR dialect is being referenced. And -emit-mlir=llvm should be used to target the LLVM dialect.

For the case where -emit-mlir=llvm -fclangir-direct-lowering is used, it should directly lower from CIR to LLVM, with -fclangir-direct-lowering remaining an ON by default. And -emit-mlir=llvm -fno-clangir-direct-lowering would proceed through the standard dialects.

[bcardosolopes]

As you add this flag, might be worth editing the one above to explain that that does to standard dialects instead.

[bcardosolopes]

I like @xlauko idea, couple things to add to the discussion:

• emit-mlir should be acceptable without additional arguments and default to std for no-direct and llvm for direct.
• We need driver tests as part of the PR (check the produced cc1 invocation).
• We need a warning if -emit-mlir=std is used with -fclangir-direct-lowering