Reparenting: adjust name lookup for associated types
We don't consider an associated type of a @reparentable protocol
to be an overridden decl of any protocols inheriting from it.
If we did, then it'd affect how getReducedType and other bits of
comparison between associated types work, because they prefer
anchors, i.e., associated types not overriding anything else
(see `swift::compareAssociatedTypes`).
That worked fine until availability checking brought to light that
name lookup also relies getOverriddenDecls() to determine which
associated type decl one would be the one we refer to in a protocol.
So in this case it's not the reduced type, but rather the actual
`DeclRefTypeRepr` that name-lookup finds for `Element`, that actually
_does_ need to view that one coming from the @reparentable one as being
"overridden", or shadowed I guess, by any other protocols downstream of it.
rdar://170819864
