[ThinLTO] Clear dllimport when setting dso_local.

This is PR36686.

If a user of a library is LTOed with that library we take the
opportunity to set dso_local, but we don't clear dllimport, which
creates an invalid IR.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327408 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed