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

stdlib/core/CompilerProtocols.swift

@@ -148,8 +148,8 @@ protocol _RawOptionSet: RawRepresentable {
 }
 
 // TODO: This is an incomplete implementation of our option sets vision.
-protocol RawOptionSet : _RawOptionSet, LogicValue, Equatable
-                        /*FIXME: , BitwiseOperations*/ {
+protocol RawOptionSet : _RawOptionSet, LogicValue, Equatable,
+                        NilLiteralConvertible {
   // A non-failable version of RawRepresentable.fromRaw.
   class func fromMask(raw: RawType) -> Self
 
@@ -162,6 +162,12 @@ protocol RawOptionSet : _RawOptionSet, LogicValue, Equatable
   /* func getLogicValue() -> Bool { return toRaw() != .allZeros() } */
 }
 
+/// Conforming to this protocol allows a type to be usable with the 'nil'
+/// literal.
+protocol NilLiteralConvertible {
+  class func convertFromNilLiteral() -> Self
+}
+
 protocol _BuiltinIntegerLiteralConvertible {
   class func _convertFromBuiltinIntegerLiteral(
                 value: MaxBuiltinIntegerType) -> Self