[GlobalMerge] Don't merge dllexport globals

Merging such globals loses the dllexport attribute. Add a test
to check that normal globals still are merged.

Differential Revision: https://reviews.llvm.org/D42127

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323307 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed