Always traverse GlobalVariable initializer when computing the export list

Summary:
We are always importing the initializer for a GlobalVariable.
So if a GlobalVariable is in the export-list, we pull in any
refs as well.

Reviewers: tejohnson

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19102

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267303