[MinGW] Emit typeinfo locally for dllimported classes without key functions

This fixes building Qt as shared libraries with clang in MinGW
mode; previously subclasses of the QObjectData class (in other
DLLs than the base DLL) failed to find the typeinfo symbols
(that neither were emitted in the base DLL nor in the DLL
containing the subclass).

If the virtual destructor in the newly added testcase wouldn't
be pure (or if there'd be another non-pure virtual method),
it'd be a key function and things would work out even before this
change. Make sure to locally emit the typeinfo for these classes
as well.

This matches what GCC does in this specific testcase.

This fixes the root issue that spawned PR35146. (The difference
to GCC that is initially described in that bug still is present
though.)

Differential Revision: https://reviews.llvm.org/D42641

llvm-svn: 324059