Add support for Return in EDSCs

This CL adds the Return op to EDSCs types and emitter.
This allows generating full function bodies that can be compiled all the way
down to LLVMIR and executed on CPU.

At this point, the MLIR lacks the testing infrastructure to exercise this.
End-to-end testing of full functions written in EDSCs is left for a future CL.

PiperOrigin-RevId: 230527530