[dllimport] A non-imported class with an imported key can't have a key

Summary:
The vtable takes its DLL storage class from the class, not the key
function. When they disagree, the vtable won't be exported by the DLL
that defines the key function. The easiest way to ensure that importers
of the class emit their own vtable is to say that the class has no key
function.

Reviewers: hans, majnemer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11913

llvm-svn: 244488