Deactivate feature gate explicit_generic_args_with_impl_trait

Signed-off-by: Nick Cameron <nrc@ncameron.org>

src/test/ui/impl-trait/explicit-generic-args-with-impl-trait/const-args.rs

@@ -0,0 +1,21 @@
+// check-pass
+
+trait Usizer {
+    fn m(self) -> usize;
+}
+
+fn f<const N: usize>(u: impl Usizer) -> usize {
+    N + u.m()
+}
+
+struct Usizable;
+
+impl Usizer for Usizable {
+    fn m(self) -> usize {
+        16
+    }
+}
+
+fn main() {
+    assert_eq!(f::<4usize>(Usizable), 20usize);
+}