[ASTImporter] Extend lookup logic in class templates

During import of a class template, lookup may find a forward
declaration and structural match falsely reports equivalency
between a forward decl and a definition. The result is that
some definitions are not imported if we had imported a forward
decl previously. This patch gives a fix.

Patch by Gabor Marton!

Differential Revision: https://reviews.llvm.org/D46353

llvm-svn: 332338