[deserialization] Change SILLinking to use SILModule for linking instead of performing the linking itself.

This approach will allow us to go back to doing lazy linking if we want
to in the future.

Swift SVN r15498

include/swift/SIL/SILModule.h

@@ -294,6 +294,12 @@ public:
   bool linkFunction(SILFunction *Fun,
                     LinkingMode LinkAll=LinkingMode::LinkNormal);
 
+  /// Link in all Witness Tables in the module.
+  void linkAllWitnessTables();
+
+  /// Link in all VTables in the module.
+  void linkAllVTables();
+
   /// \brief Return the declaration of a utility function that can,
   /// but needn't, be shared between modules.
   SILFunction *getOrCreateSharedFunction(SILLocation loc,