Teach the internalize pass to skip dllexported symbols because they could be
referenced in a way that even the linker does not see.

Differential Revision: http://llvm-reviews.chandlerc.com/D2280



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196300 91177308-0d34-0410-b5e6-96231b3b80d8