Fix #24895.

[breaking-change]

What does this break?  Basically, code that implements `Drop` and is
using `T:Copy` for one of its type parameters and is relying on the
Drop Check rule not applying to it.

Here is an example:

```rust
#![allow(dead_code,unused_variables,unused_assignments)]
struct D<T:Copy>(T);
impl<T:Copy> Drop for D<T> { fn drop(&mut self) { } }

trait UserT { fn c(&self) { } }
impl<T:Copy> UserT for T { }
struct E<T:UserT>(T);
impl<T:UserT> Drop for E<T> { fn drop(&mut self) { } }

// This one will start breaking.
fn foo() { let (d2, d1); d1 = D(34); d2 = D(&d1); }

#[cfg(this_one_does_and_should_always_break)]
fn bar() { let (e2, e1); e1 = E(34); e2 = E(&e1); }

fn main() {
    foo();
}
```

src/librustc_typeck/check/dropck.rs

@@ -464,9 +464,9 @@ fn iterate_over_potentially_unsafe_regions_in_type<'a, 'tcx>(
                     ty::Predicate::Trait(ty::Binder(ref t_pred)) => {
                         let def_id = t_pred.trait_ref.def_id;
                         match rcx.tcx().lang_items.to_builtin_kind(def_id) {
+                            // Issue 24895: deliberately do not include `BoundCopy` here.
                             Some(ty::BoundSend) |
                             Some(ty::BoundSized) |
-                            Some(ty::BoundCopy) |
                             Some(ty::BoundSync) => false,
                             _ => true,
                         }