add the leak check to the new solver

tests/ui/higher-ranked/leak-check-in-selection.rs

@@ -0,0 +1,24 @@
+// run-pass
+// revisions: old next
+//[next] compile-flags: -Ztrait-solver=next
+#![allow(coherence_leak_check)]
+
+trait Trait: Sized {
+    fn is_higher_ranked(self) -> bool;
+}
+
+impl Trait for for<'a> fn(&'a ()) {
+    fn is_higher_ranked(self) -> bool {
+        true
+    }
+}
+impl<'a> Trait for fn(&'a ()) {
+    fn is_higher_ranked(self) -> bool {
+        false
+    }
+}
+
+fn main() {
+    let x: for<'a> fn(&'a ()) = |&()| ();
+    assert!(x.is_higher_ranked());
+}