ModuleLinker: Do not import linkonce/weak as "external_weak"

Summary:
There is no reason to have a weak reference because the external
definition will be weak.

Reviewers: rafael

Subscribers: llvm-commits, tejohnson

Differential Revision: http://reviews.llvm.org/D19267

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266750