Use NoGen plan for unboxed-tuple bindings

There was a small mixup here, exposed by Trac #8762.
Now clarified with better function names and comments.
Allow ($) to return an unlifted type (Trac #8739)

Since ($) simply returns its result, via a tail call, it can
perfectly well have an unlifted result type; e.g.
    foo $ True    where  foo :: Bool -> Int#
should be perfectly fine.

This used to work in GHC 7.2, but caused a Lint failure.  This patch
makes it work again (which involved removing code in TcExpr), but fixing
the Lint failure meant I had to make ($) into a wired-in Id.  Which
is not hard to do (in MkId).