Add missing file from r204570.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204574 91177308-0d34-0410-b5e6-96231b3b80d8
Treat dllimport globals without explicit storage class as extern

dllimport implies a definition which means the 'extern' keyword is optional
when declaring imported variables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204576 91177308-0d34-0410-b5e6-96231b3b80d8