Don't emit structors for available_externally globals (PR19933)

We would previously assert here when trying to figure out the section
for the global.

This makes us handle the situation more gracefully since the IR isn't
malformed.

Differential Revision: http://reviews.llvm.org/D4022

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210215 91177308-0d34-0410-b5e6-96231b3b80d8