[DebugInfo][InstrRef][NFC] Emit variadic debug values from InstrRefLDV

In preparation for supporting DBG_VALUE_LIST in InstrRefLDV, this patch
adds the logic for emitting DBG_VALUE_LIST instructions from
InstrRefLDV. The logical changes here are fairly simple, with the main
change being that instead of directly prepending offsets to the DIExpr,
we use appendOpsToArg to modify the expression for individual debug
operands in the expression. The function emitLoc is also changed to take
a list of debug ops, with an empty list meaning an undef value.

Differential Revision: https://reviews.llvm.org/D128209