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.

CommandLineArguments.cxx

@@ -66,7 +66,7 @@ class CommandLineArgumentsMapOfStrucs
 class CommandLineArgumentsInternal
 {
 public:
-  CommandLineArgumentsInternal() {}
+  CommandLineArgumentsInternal() = default;
 
   using VectorOfStrings = CommandLineArgumentsVectorOfStrings;
   using CallbacksMap = CommandLineArgumentsMapOfStrucs;