[Win32 ABI] Defer operator delete checks until vtable is marked used

We were previously checking at every destructor declaration, but that was a bit
excessive. Since the deleting destructor is emitted with the vtable, do the
check when the vtable is marked used.

Differential Revision: http://llvm-reviews.chandlerc.com/D2851

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@202046 91177308-0d34-0410-b5e6-96231b3b80d8