convert AdtDef::destructor to on-demand

This removes the Cell from AdtDef. Also, moving destructor validity
checking to on-demand (forced during item-type checking) ensures that
invalid destructors can't cause ICEs.

Fixes #38868.
Fixes #40132.

src/test/compile-fail/issue-38868.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub struct List<T> {
+    head: T,
+}
+
+impl Drop for List<i32> { //~ ERROR E0366
+    fn drop(&mut self) {
+        panic!()
+    }
+}
+
+fn main() {
+    List { head: 0 };
+}