[mlir] Support FuncOpSignatureConversion for more FunctionLike ops.

This extracts the implementation of getType, setType, and getBody from
FunctionSupport.h into the mlir::impl namespace and defines them
generically in FunctionSupport.cpp. This allows them to be used
elsewhere for any FunctionLike ops that use FunctionType for their
type signature.

Using the new helpers, FuncOpSignatureConversion is generalized to
work with all such FunctionLike ops. Convenience helpers are added to
configure the pattern for a given concrete FunctionLike op type.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D95021