CodeGen: tweak CFString emission for COFF targets

The `isa' member was previously not given the correct DLL Storage.  Ensure that
we give the `isa' constant `__CFConstantStringClassReference' the correct DLL
storage.  Default to dllimport unless an explicit specification gives it a
dllexport storage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271361 91177308-0d34-0410-b5e6-96231b3b80d8