Unary move breaks linearity when moving out of enums

[bblum]

This code compiles and runs the destructor twice:

class noncopyable {
    i: (); new() { self.i = (); } drop { #error["dropped"]; }
}
enum wrapper = noncopyable;

fn main() {
    let x1 = wrapper(noncopyable());
    let x2 = move *x1;
}

If it's written let x2 <- *x1, it instead errors with error: moving out of enum content, which is perhaps what this should do too.

[bblum]

Given where that other compile error comes from, I think this is borrowck's job.

[nikomatsakis]

dup of #3071 I believe