Don't drop dllimport from qualified friend redeclarations (PR20512)

This matches MSVC's logic, which seems to be that when the friend
declaration is qualified, it cannot be a declaration of a new symbol
and so the dll linkage doesn't change.

Differential Revision: http://reviews.llvm.org/D4764

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214774 91177308-0d34-0410-b5e6-96231b3b80d8