Implementation and testing for poisoning vtable

ptr in dtor.

Summary:
After destruction, invocation of virtual functions prevented
by poisoning vtable pointer.

Reviewers: eugenis, kcc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D12712

Fixed testing callback emission order to account for vptr.
Poison vtable in either complete or base dtor, depending on
if virtual bases exist. If virtual bases exist, poison in
complete dtor. Otherwise, poison in base.
Remove commented-out block.

llvm-svn: 247762