swift234

Implement <rdar://problem/16951729> nil should be a literal type

This is all goodness, and eliminates a major source of implicit conversions.
One thing this regresses on though, is that we now reject "x == nil" where
x is an option type and the element of the optional is not Equtatable.  If
this is important, there are ways to enable this, but directly testing it as
a logic value is more straight-forward.

This does not include support for pattern matching against nil, that will be
a follow on patch.



Swift SVN r18918