[clang][ASTImporter] Handle UsingType in friend declarations.

ASTImporterLookupTable did not handle correctly friend declarations
where the friend type is an UsingType (type of a declaration that
comes from an using-declaration). The problem is fixed by handling
it in the same way as a friend with TypedefType.

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D142607