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

lib/Bitcode/Reader/ValueList.cpp

@@ -58,7 +58,7 @@ void BitcodeReaderValueList::assignValue(Value *V, unsigned Idx) {
   if (Idx >= size())
     resize(Idx + 1);
 
-  WeakVH &OldV = ValuePtrs[Idx];
+  WeakTrackingVH &OldV = ValuePtrs[Idx];
   if (!OldV) {
     OldV = V;
     return;