[modules] When instantiating a class member, don't expect to find the previous
declaration in the instantiation if the previous declaration came from another
definition of the class template that got merged into the pattern definition.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219552 91177308-0d34-0410-b5e6-96231b3b80d8