Fix Trac #2246; overhaul handling of overloaded literals

The real work of fixing Trac #2246 is to use shortCutLit in
MatchLit.dsOverLit, so that type information discovered late in the
day by the type checker can still be exploited during desugaring.

However, as usual I found myself doing some refactoring along the
way, to tidy up the handling of overloaded literals.   The main
change is to split HsOverLit into a record, which in turn uses
a sum type for the three variants.  This makes the code significantly
more modular.

data HsOverLit id
  = OverLit {
	ol_val :: OverLitVal,
	ol_rebindable :: Bool,		-- True <=> rebindable syntax
					-- False <=> standard syntax
	ol_witness :: SyntaxExpr id,	-- Note [Overloaded literal witnesses]
	ol_type :: PostTcType }

data OverLitVal
  = HsIntegral   !Integer   	-- Integer-looking literals;
  | HsFractional !Rational   	-- Frac-looking literals
  | HsIsString   !FastString 	-- String-looking literals