Support value-typed references in iterator facade's operator->()

Add a PointerProxy similar to the existing iterator_facade_base::ReferenceProxy and return it from the arrow operator. This prevents iterator facades with a reference type that is not a true reference to take the address of a temporary.

Forward the reference type of the mapped_iterator to the iterator adaptor which in turn forwards it to the iterator facade. This fixes mlir::op_iterator::operator->() to take the address of a temporary.

Make some polishing changes to op_iterator and op_filter_iterator.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D109490