[DebugInfo] Support representation of multiple location operands in SDDbgValue

This patch modifies the class that represents debug values during ISel,
SDDbgValue, to support multiple location operands (to represent a dbg.value that
uses a DIArgList). Part of this class's functionality has been split off into a
new class, SDDbgOperand.

The new class SDDbgOperand represents a single value, corresponding to an SSA
value or MachineOperand in the IR and MIR respectively. Members of SDDbgValue
that were previously related to that specific value (as opposed to the
variable or DIExpression), such as the Kind enum, have been moved to
SDDbgOperand. SDDbgValue now contains an array of SDDbgOperand instead, allowing
it to hold more than one of these values.

All changes outside SDDbgValue are simply updates to use the new interface.

Differential Revision: https://reviews.llvm.org/D88585