conform to LogicValue.
This approach was taken to keep _isNull because I first tried
to just use comparisons to nil instead of isNull(). Apparently
that led to some circular definitions, so it was easier to just
stage it this way.
Swift SVN r18301
It is replaced by debugPrint() family of functions, that are called by REPL.
There is a regression in printing types that don't conform to Printable, this
is tracked by rdar://16898708
Swift SVN r18006
