Rename WeakVH to WeakTrackingVH; NFC

Summary:
I plan to use WeakVH to mean "nulls itself out on deletion, but does
not track RAUW" in a subsequent commit.

Reviewers: dblaikie, davide

Reviewed By: davide

Subscribers: arsenm, mehdi_amini, mcrosier, mzolotukhin, jfb, llvm-commits, nhaehnle

Differential Revision: https://reviews.llvm.org/D32266

llvm-svn: 301424