Propagate DLLAttr to friend re-declarations of member functions

...that have already been constructed (e.g., in inner classes) while parsing the
class definition.  They would otherwise lack any DLLAttr inherited from the
class, which are only set here (called from Sema::CheckCompletedClass) after the
class definition has been parsed completely.

Differential revision: https://reviews.llvm.org/D16632

llvm-svn: 326990