Fix an ICE when implementing Drop on enums

[ghost]

Fixes #13041.

[alexcrichton]

Could you add a test to ensure that this doesn't leak memory?

enum Foo { NoDrop, NeedsDrop(Box<int>) }
impl Drop for Foo {
    fn drop(&mut self) {
        *self = NeedsDrop(box 3);
    }
}

[ghost]

@alexcrichton Good call, this needs more work. :-) What was I thinking? Will reopen.