[modules] Sometimes we can deserialize a class member but not have yet

encountered any definition for the class; this happens when the definition is
added by an update record that is not yet loaded. In such a case, eagerly pick
the original parent of the member as the canonical definition of the class
rather than muddling through with the canonical declaration (the latter can
lead to us failing to merge properly later if the canonical definition turns
out to be some other declaration).

llvm-svn: 226977