[PM] Introduce a PoisoningVH as a (more expensive) alternative to

AssertingVH that delays any reported error until the handle is *used*.

This allows data structures to contain handles which become dangling
provided the data structure is cleaned up afterward rather than used for
anything interesting.

The implementation is moderately horrible in part because it works to
leave AssertingVH in place, undisturbed. If at some point there is
consensus that this is simply how AssertingVH should be used, it can be
substantially simplified.

This remains a boring pointer in a non-asserts build as you would
expect. The only place we pay cost is in asserts builds.

I plan to use this as a basis for replacing the asserting VHs that
currently dangle in the new PM until invalidation occurs in both LVI and
SCEV.

Differential Revision: https://reviews.llvm.org/D29061

llvm-svn: 292925