Add defaulted copy and move operations to format_error and system_error (#1347)

* Avoid weak vtables by providing a private virtual member function

* Add warning Wweak-vtables to clang when FMT_PEDANTIC is on

* Add defaulted copy and move operations to format_error and system_error

Compiler generated copy operations are deprecated and move operations
are not generated altogether.

* Add warning Wdeprecated to clang when FMT_PEDANTIC is on

test/test-assert.h

@@ -14,8 +14,13 @@
 class assertion_failure : public std::logic_error {
  public:
   explicit assertion_failure(const char* message) : std::logic_error(message) {}
+
+ private:
+  virtual void avoid_weak_vtable();
 };
 
+void assertion_failure::avoid_weak_vtable() {}
+
 #define FMT_ASSERT(condition, message) \
   if (!(condition)) throw assertion_failure(message);