Commit to a primary definition for a class when we load its first

member.

Previously, we wouldn't do this if the first member loaded is within a
definition that's added to a class via an update record, which happens
when template instantiation adds a class definition to a declaration
that was imported from an AST file.

This would lead to classes having member functions whose getParent
returned a class declaration that wasn't the primary definition, which
in turn caused the vtable builder to build broken vtables.

I don't yet have a reduced testcase for the wrong-code bug here, because
the setup required to get us into the broken state is very subtle, but
have confirmed that this fixes it.