[Accessibility] Protocol witnesses must be as accessible as the protocol.

...unless the type has less accessibility than the protocol, in which case
they must be as accessible as the type.

This restriction applies even with access control checking disabled, but
shouldn't affect any decls not already marked with access control modifiers.

Swift SVN r19382

stdlib/core/CompilerProtocols.swift

@@ -174,7 +174,7 @@
   class func convertFromNilLiteral() -> Self
 }
 
-@internal protocol _BuiltinIntegerLiteralConvertible {
+@public protocol _BuiltinIntegerLiteralConvertible {
   class func _convertFromBuiltinIntegerLiteral(
                 value: MaxBuiltinIntegerType) -> Self
 }
@@ -184,7 +184,7 @@
   class func convertFromIntegerLiteral(value: IntegerLiteralType) -> Self
 }
 
-@internal protocol _BuiltinFloatLiteralConvertible {
+@public protocol _BuiltinFloatLiteralConvertible {
   class func _convertFromBuiltinFloatLiteral(
                 value: MaxBuiltinFloatType) -> Self
 }