Update CodeGen to use DLL storage class for dllimport/dllexport

With the old linkage types removed, set the linkage to external for both
dllimport and dllexport to reflect what's currently supported.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199220 91177308-0d34-0410-b5e6-96231b3b80d8