[serialization] Validate the control block -- is the module too new?

Not so interesting right now, but eventually we'll be validating other
things as well, and it's best not to have FIXMEs lying around.

Swift SVN r5175

include/swift/Serialization/SerializedModuleLoader.h

@@ -50,13 +50,6 @@ public:
   /// emits a diagnostic and returns NULL.
   virtual Module *
   loadModule(SourceLoc importLoc, Module::AccessPathTy path) override;
-
-private:
-  /// Report an error in reading a module file.
-  ///
-  /// \returns A dummy module, to prevent reloadeding 
-  // FIXME: This should move to a dedicated Reader class.
-  Module *error(std::pair<Identifier, SourceLoc> importLoc);
 };
 
 }