Prevent ICE in dllexport class with _Atomic data member

Guard against a null pointer dereference that caused Clang to crash
when processing a class containing an _Atomic qualified data member,
and that is tagged with 'dllexport'.

Differential Revision: https://reviews.llvm.org/D29208


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293911 91177308-0d34-0410-b5e6-96231b3b80d8