Don't trigger missing_const_for_fn in external macros

As reported in #3841. Only fixes the part where it triggers on the
`derive`.

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -55,3 +55,7 @@ trait Foo {
         33
     }
 }
+
+// Don't lint in external macros (derive)
+#[derive(PartialEq, Eq)]
+struct Point(isize, isize);