[LiveDebugValues][nfc] Reduce memory usage of InstrRef (#76051)

Commit 1b531d54 (#74203) removed the usage of unique_ptrs of arrays
in favour of using vectors, but inadvertently increased peak memory
usage by removing the ability to deallocate vector memory that was no
longer needed mid-LDV.

In that same review, it was pointed out that `FuncValueTable` typedef
could be removed, since it was "just a vector".

This commit addresses both issues by making `FuncValueTable` a real data
structure, capable of mapping BBs to ValueTables and able to free
ValueTables as needed.

This reduces peak memory usage in the compiler by 10% in the benchmarks
flagged by the original review.

As a consequence, we had to remove a handful of instances of the
"declare-then-initialize" antipattern in unittests, as the
FuncValueTable class is no longer default-constructible.