Sema: Fix some issues with overrides of materializeForSet
When overriding storage with a forced static dispatch materializeForSet,
the override's materializeForSet should not override the base
materializeForSet.
This is the case where a dynamic property witnesses a protocol
requirement, and Sema synthesizes a materializeForSet for it.
In this case, the synthesized materializeForSet dynamically dispatches
to the dynamic property's getter and setter, and the protocol witness
thunk directly calls the synthesized materializeForSet.
The subclass only needs to override the getter and setter in this case,
since the base class's materializeForSet will already do the right
thing.
In fact, marking it as an override exposes a problem where we cannot
serialize an xref to an imported property's materializeForSet, since
it was not created by the importer.
