Usually this happens directly, through some use of the class and its
conformance. However, if a conformance is /only/ used to satisfy an
associated type, we seem to bypass the step that actually infers
selector names for accessors, even though we do it successfully for
methods. Fix this by making sure the accessor decls are validated when
a property is, something that normal uses of a property probably have
to do anyway.
Also, simplify inferObjCName by assuming/asserting that it is only
used on things that are already marked @objc.
https://bugs.swift.org/browse/SR-6944
