[DebugInfo] Add support for variadic DBG_INSTR_REFs in LiveDebugValues

Following support from the previous patches in this stack being added for
variadic DBG_INSTR_REFs to exist, this patch modifies LiveDebugValues to
handle those instructions. Support already exists for DBG_VALUE_LISTs, which
covers most of the work needed to handle these instructions; this patch only
modifies the transferDebugInstrRef function to correctly track them.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133927