[DebugInfo][InstrRef][NFC] Use unique_ptr instead of raw pointers

InstrRefBasedLDV allocates some big tables of ValueIDNum, to store live-in
and live-out block values in, that then get passed around as pointers
everywhere. This patch wraps the allocation in a std::unique_ptr, names
some types based on unique_ptr, and passes references to those around
instead. There's no functional change, but it makes it clearer to the
reader that references to these tables are borrowed rather than owned, and
we get some extra validity assertions too.

Differential Revision: https://reviews.llvm.org/D118774