dump-decls: Suppress unit-ids

While the testsuite driver already normalizes these away, they are
nevertheless a severe nuisance when diffing outside of the testsuite.

Intriguingly, this doesn't completely eliminate the unit IDs; some
wired-in names are still printed. However, this is a cheap and helpful
improvement over the status quo so I am simply going to accept this.

Fixes #25334.