[DebugInfo][InstrRef] Handle transfers of variadic debug values in LDV

This patch adds the last of the changes required to enable
DBG_VALUE_LIST handling in InstrRefLDV, handling variadic debug values
during the transfer tracking step. Most of the changes are fairly
straightforward, and based around tracking multiple locations per
variable in TransferTracker::VLocTracker.

Differential Revision: https://reviews.llvm.org/D128211