Managed boxes are not completely gated

[alexcrichton]

This program successfully compiles when it shouldn't:

fn main() { @3; }

[thestinger]

This is also how once functions are gated, so that probably needs to be changed too.

[alexcrichton]

Taking a look at the AST, Onceness only shows up in TyClosure, so I think that it's correct for once functions, it just appears that box literals aren't handled.