[MinGW] Produce a vtable and RTTI for dllexported classes without a key function

This matches what GCC does in these situations.

This fixes compiling Qt in debug mode. In release mode, references to
the vtable of this particular class ends up optimized away, but in debug
mode, the compiler creates references to the vtable, which is expected
to be dllexported from a different DLL. Make sure the dllexported
version actually ends up emitted.

Differential Revision: https://reviews.llvm.org/D55698

llvm-svn: 349256