-Wunused-pattern-binds: Recurse into patterns to check whether there's a splice

See the examples in #22057 which show we have to traverse deeply into a
pattern to determine whether it contains a splice or not. The original
implementation pointed this out but deemed this very shallow traversal
"too expensive".

Fixes #22057

I also fixed an oversight in !7821 which meant we lost a warning which
was present in 9.2.2.

Fixes #22067