Don't create hidden dllimport global values.

Hidden visibility is almost the opposite of dllimport. We were
producing them before (dllimport wins in the existing llvm
implementation), but now the llvm verifier produces an error.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@323361 91177308-0d34-0410-b5e6-96231b3b80d8