Add suppression if nowarn differs (scala#23652)

Fixes scala#23651 

The [previous fix](scala#22383) for the
same `@nowarn` attached to multiple elements should have compared the
`annotPos` to identify duplicates (instead of the target range).

This commit defers detecting "bad" or duplicate suppressions (which
originate with the same annotation) to report time, after the
suppression is "unused"; there are few nowarns per file and fewer that
are unused.

While checking for a suppression, mark matching unused suppressions as
"superseded", so that if they remain unused, the warning can add an
"audit" that the nowarn matched a diagnostic (but was superseded by some
other nowarn).

~This commit goes further and checks for duplicates (including whether
the filters look the same).~

~If it finds a duplicate where the `annotPos` differs, warn about the
user-written annotation.~

~Filters match each other if they are the same type and, if they have a
pattern, the string representations of the patterns are equal.~
[Cherry-picked 40843f7][modified]

Author: tgodzik