Unused move causes a crash

[jruderman]

fn main()
{
    let y = ~1;
    move y;
}

a.out(43383,0x10065f000) malloc:
*** error for object 0x7ffed14144d0: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

[nikomatsakis]

I think the problem is that, when translating expr_unary_move, if the dest is "ignore", we drop the lvalue but do not zero it.

[vertexclique]

Which fn we use instead of drop_val( lvalue ) in "Ignore" case?

[catamorphism]

This was merged in already, but I also added the test case:

9d276b1

[vertexclique]

@catamorphism I see him there src/test/run-pass/unused-move.rs