Introduce "inherited" default arguments and use them for inherited initializers.

Previously, we were cloning the default arguments completely, which
meant code duplication (when inheriting within a module) or simply a
failure (when inheriting across modules). Now, we reference the
default arguments where we inherited them, eliminating the
duplication. Part of <rdar://problem/16318855>.

Swift SVN r15062

include/swift/Serialization/ModuleFormat.h

@@ -177,6 +177,7 @@ enum DefaultArgumentKind : uint8_t {
   Line,
   Column,
   Function,
+  Inherited,
 };
 using DefaultArgumentField = BCFixed<3>;