[RemoveDI] Handle DPValues in SROA (#74089)

Handle dbg.declares in SROA using DPValues.

In order to reduce duplication, the migrate-debug-info loop has been changed
to a generic lambda with some helper function overloads, which is called
for dbg.declares, dbg.assigns, and DPValues alike.

The tests will become "live" once #74090 lands (see for more info).