Don't ICE when implementing Drop on enums

[brson]

cc #6250

Drop not only doesn't work on enums, it makes the compiler crash. Nominating.

[pnkfelix]

We will accept ICE's, reluctantly, in a 1.0 release.

Therefore, not a 1.0 blocker. Assigning P-low for now.

[lilyball]

Minimal repro:

enum Foo {
    Bar
}

impl Drop for Foo {
    fn drop(&mut self) {}
}

ICE message:

error: internal compiler error: ID not mapped to struct fields: enum Foo::Foo (id=4)