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).
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