[LiveDebugValues] Encode a location in VarLoc IDs, NFC [2/3]

This is part 2 of a 3-part series to address a compile-time explosion
issue in LiveDebugValues.

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Each VarLoc has a unique ID: this ID is used to look up a VarLoc in the
VarLocMap, and to virtually insert a VarLoc into a VarLocSet. Instead of
inserting the VarLoc /itself/ into the VarLocSet, we insert just the ID,
because this can be represented efficiently with a SparseBitVector.

This change introduces LocIndex, a layer of abstraction on top of VarLoc
IDs. Prior to this change, an ID was just an index into a vector. With
this change, an ID encodes both an index /and/ a register location. The
type-checker ensures that conversions to and from LocIndex are correct.

For the moment the register location is always 0 (undef). We have plenty
of bits left over to encode physregs, stack slots, and other locations
in the future.

Differential Revision: https://reviews.llvm.org/D74985