Duplicated ABIInfo, RequiredArgs, etc

[bcardosolopes]

From @smeenai in #1025 (comment)_

... we have three symbols duplicated between ::cir and ::mir:cir:

• cir::ABIInfo and mlir::CIR::ABIInfo
• cir::RequiredArgs and mlir::cir::RequiredArgs
• cir::ReturnValueSlot and mlir::cir::ReturnValueSlot

The cir ones are all in clang/lib/CIR/CodeGen, and the mlir::cir ones are all in clang/lib/CIR/Dialect/Transforms/TargetLowering.

We should get rid of this duplication, it's tech debt from gsoc (cc @sitio-couto)