[serialization] Serialize protocol conformances instead of ignoring them.

For some reason this didn't actually affect any testing, but it's going to
be necessary very soon.

Swift SVN r5987

lib/Serialization/ModuleFile.h

@@ -35,6 +35,7 @@ class DeclContext;
 class GenericParamList;
 class Module;
 class Pattern;
+class ProtocolConformance;
 class ValueDecl;
 
 /// Describes whether a loaded module can be used.
@@ -142,6 +143,11 @@ private:
   /// If the record at the cursor is not a pattern, returns null.
   Pattern *maybeReadPattern();
 
+  /// Recursively reads a protocol conformance from \c DeclTypeCursor.
+  ///
+  /// If the record at the cursor is not a protocol conformance, returns null.
+  ProtocolConformance *maybeReadConformance();
+
   /// Reads a generic param list from \c DeclTypeCursor.
   ///
   /// If the record at the cursor is not a generic param list, returns null