STYLE: Prefer = default to explicitly trivial implementations

This check replaces default bodies of special member functions with
= default;. The explicitly defaulted function declarations enable more
opportunities in optimization, because the compiler might treat
explicitly defaulted functions as trivial.

Additionally, the C++11 use of = default more clearly expreses the
intent for the special member functions.

Following the change in ITK, e.g.:
InsightSoftwareConsortium/ITK@5142ed7

src/Developer/ImageSource.h

@@ -25,8 +25,8 @@ class ImageSource : public ProcessObject
   itkTypeMacro(ImageSource, ProcessObject);
 
 protected:
-  ImageSource() {}
-  ~ImageSource() {}
+  ImageSource() = default;
+  ~ImageSource() = default;
 
   /** Does the real work. */
   virtual void