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

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

llvm-svn: 220532