[GlobalMerge] Allow merging of dllexported variables

If merging them, the dllexport attribute needs to be brought along
to the new GlobalAlias.

Differential Revision: https://reviews.llvm.org/D43192

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324937 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed