Don't emit strong vtable definitions for imported classes with key functions (PR21355)

Clang would previously assert on the following code when targeting MinGW:

  struct __declspec(dllimport) S {
      virtual ~S();
  };
  S::~S() {}

Because ~S is a key function and the class is dllimport, we would try to emit a
strong definition of the vtable, with dllimport - which is a conflict. We
should not emit strong vtable definitions for imported classes.

Differential Revision: http://reviews.llvm.org/D5944