Serialization: introduce a conformance cross-reference so we don’t serialize normal protocol conformances in modules that didn’t declare them.

Swift SVN r20792

include/swift/Serialization/ModuleFormat.h

@@ -40,7 +40,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// Serialized module format minor version number.
 ///
 /// When the format changes IN ANY WAY, this number should be incremented.
-const uint16_t VERSION_MINOR = 123;
+const uint16_t VERSION_MINOR = 124;
 
 using DeclID = Fixnum<31>;
 using DeclIDField = BCFixed<31>;
@@ -930,6 +930,13 @@ namespace decls_block {
     BCFixed<1>                 // dummy
   >;
 
+  using XRefProtocolConformanceLayout = BCRecordLayout<
+    XREF_PROTOCOL_CONFORMANCE,
+    DeclIDField,    // protocol
+    DeclIDField,    // nominal type
+    ModuleIDField   // module
+  >;
+
   /// A placeholder for lack of conformance information. Conformances are
   /// indexed, so simply omitting one would be incorrect.
   using NoConformanceLayout = BCRecordLayout<