Fix non-determinism in debuginfo (#68332)

Assignment tracking iterates over a SmallSet when adding metadata, which
eventually results in debug metadata being added to the module in
non-deterministic order.

As reported in #63921, we saw some cases where DWARF DebugLoc entries
could have their order reversed, even though there was no functional
difference.

This patch replaces the `SmallSet` with a `SmallVector`, and adds the
required `DenseMapInfo` specialization to make the ordering
deterministic.

Fixes #63921