pprHsArrow does not use its IsUnicodeSyntax information to print.

!4224 (merged) introduces pprHsArrow, as a common point for pretty printing all HsArrow occurrences.

But it also adds a IsUnicodeSyntax field to each arrow type, captured in the parser.

The current implementation prints in normal or unicode syntax according to DynFlags settings.

Change it to honour the IsUnicodeSyntax field instead.