Implement -f[no-]print-unicode-syntax flag for unicode syntax output (#8959)

There is currently no way to separate whether UnicodeSyntax is accepted
for input from the corresponding output syntax using unicode symbols.
This patch implements a separate flag for affecting ghc(i)'s output.

Signed-off-by: Bertram Felgenhauer <int-e@gmx.de>

Reviewed By: nomeata, austin

Differential Revision: https://phabricator.haskell.org/D807

GHC Trac Issues: #8959