Ignore type bindings in generic_predicates_for_param

This allows us to handle more cases without a query cycle, which
includes certain cases that rustc accepted. That in turn means we avoid
triggering salsa-rs/salsa#257 on valid code (it will still happen if the
user writes an actual cycle).

We actually accept more definitions than rustc now; that's because rustc
only ignores bindings when looking up super traits, whereas we now also
ignore them when looking for predicates to disambiguate associated type
shorthand. We could introduce a separate query for super traits if
necessary, but for now I think this should be fine.

crates/hir/src/lib.rs

@@ -2068,7 +2068,10 @@ impl Type {
                 match pred {
                     WhereClause::Implemented(trait_ref) => {
                         cb(type_.clone());
-                        walk_substs(db, type_, &trait_ref.substitution, cb);
+                        // skip the self type. it's likely the type we just got the bounds from
+                        for ty in trait_ref.substitution.iter().skip(1) {
+                            walk_type(db, &type_.derived(ty.clone()), cb);
+                        }
                     }
                     _ => (),
                 }