Emulate TrackingVH using WeakVH

Emulate TrackingVH using WeakVH

Summary:
This frees up one slot in the HandleBaseKind enum, which I will use
later to add a new kind of value handle.  The size of the
HandleBaseKind enum is important because we store a HandleBaseKind in
the low two bits of a (in the worst case) 4 byte aligned pointer.

Reviewers: davide, chandlerc

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D32634

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301809 91177308-0d34-0410-b5e6-96231b3b80d8