Don't speculatively emit VTTs for classes unless we are able to correctly emit...

Don't speculatively emit VTTs for classes unless we are able to correctly emit references to all the functions they will (directly or indirectly) reference.

Summary:
This fixes a miscompile where we'd emit a VTT for a class that ends up
referencing an inline virtual member function that we can't actually
emit a body for (because we never instantiated it in the current TU),
which in a corner case of a corner case can lead to link errors.

Reviewers: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54768

llvm-svn: 347692