[Serialization] Avoid serializing source orders, trust deserialized decls in preserving such order. rdar://24610992

Thank Jordan for suggesting this.

include/swift/AST/Module.h

@@ -675,11 +675,6 @@ public:
     return None;
   }
 
-  virtual Optional<unsigned>
-  getSourceOrderForDecl(const Decl *D) const {
-    return None;
-  }
-
   virtual void collectAllGroups(std::vector<StringRef> &Names) const {}
 
   /// Returns an implementation-defined "discriminator" for \p D, which