[MIR] Add hair::Constant<'tcx> and change ExprKind::Repeat to use it

[nagisa]

See nagisa@234c5c3#commitcomment-14339085 and relevant FIXME in src/librustc_mir/hair/mod.rs

[nikomatsakis]

As a side-note, http://smallcultfollowing.com/babysteps/blog/2015/08/20/virtual-structs-part-3-bringing-enums-and-structs-together/ would solve this problem quite neatly, if we were to adopt and implement it. :)