Sweep out some extra warnings

Swept:
`-Wpadded` in some places (where it caused extra size, instead of just
saying "hey, we padded struct at the end to align, just as standard says")
`-Wweak-vtables` everywhere (Clang)
`-Wexit-time-destructors` everywhere (Clang)
`-Wmissing-noreturn` everywhere (Clang)

The last three are enabled for Clang compilation going forward.

Also enabled `-Wunreachable-code` for Clang and GCC

include/internal/catch_registry_hub.cpp

@@ -10,7 +10,7 @@
 
 #include "catch_context.h"
 #include "catch_test_case_registry_impl.h"
-#include "catch_reporter_registry.hpp"
+#include "catch_reporter_registry.h"
 #include "catch_exception_translator_registry.h"
 #include "catch_tag_alias_registry.h"
 #include "catch_startup_exception_registry.h"
@@ -23,8 +23,7 @@ namespace Catch {
                             private NonCopyable {
 
         public: // IRegistryHub
-            RegistryHub() {
-            }
+            RegistryHub() = default;
             IReporterRegistry const& getReporterRegistry() const override {
                 return m_reporterRegistry;
             }