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_common.h

@@ -36,8 +36,8 @@ namespace Catch {
         NonCopyable& operator = ( NonCopyable && )     = delete;
 
     protected:
-        NonCopyable()          = default;
-        virtual ~NonCopyable() = default;
+        NonCopyable();
+        virtual ~NonCopyable();
     };
 
     struct SourceLineInfo {