Add a 'WitnessMethod' enumerator to AbstractCC.

We'll use this to represent the slightly-different polymorphic calling convention used for passing Self into protocol witnesses.

Swift SVN r10898

lib/Serialization/ModuleFormat.h

@@ -69,9 +69,10 @@ enum AbstractCC : uint8_t {
   C = 0,
   ObjCMethod,
   Freestanding,
-  Method
+  Method,
+  WitnessMethod,
 };
-using AbstractCCField = BCFixed<2>;
+using AbstractCCField = BCFixed<3>;
 
 // These IDs must \em not be renumbered or reordered without incrementing
 // VERSION_MAJOR.