[ASTImporter] Fix for import of friend class template with definition.

Summary:
If there is a friend class template "prototype" (forward declaration)
and later a definition for it in the existing code, this existing
definition may be not found by ASTImporter because it is not linked
to the prototype (under the friend AST node). The problem is fixed by
looping over all found matching decls instead of break after the first
found one.

Reviewers: martong, a.sidorin, shafik, a_sidorin

Reviewed By: a_sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65269

llvm-svn: 368551