Add a new WeakVH value handle; NFC

Summary:
WeakVH nulls itself out if the value it was tracking gets deleted, but
it does not track RAUW.

Reviewers: dblaikie, davide

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D32267

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301425 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed