Add outfuncs.c support for RawStmt nodes.

Add outfuncs.c support for RawStmt nodes.

I noticed while poking at a report from Andrey Lepikhov that the
recent addition of RawStmt nodes at the top of raw parse trees
makes it impossible to print any raw parse trees whatsoever,
because outfuncs.c doesn't know RawStmt and hence fails to descend
into it.

While we generally lack outfuncs.c support for utility statements,
there is reasonably complete support for what you can find in a
raw SELECT statement.  It was not my intention to make that all
dead code ... so let's add support for RawStmt.

Back-patch to v10 where RawStmt appeared.