[Assignment Tracking][NFC] Use RawLocationWrapper in VarLocInfo [2/x]

Use RawLocationWrapper rather than a Value to represent the location operand(s)
so that it's possible to represent multiple location
operands. AssignmentTrackingAnalysis still converts variadic debug intrinsics
to kill locations so this patch is NFC.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D145911