-Wunused-pattern-binds: Recurse into patterns to check whether there's a splice
See the examples in #22057 (closed) 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 (closed)
I also fixed an oversight in !7821 (closed) which meant we lost a warning which was present in 9.2.2.
Fixes #22067 (closed)