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.cpp

@@ -46,4 +46,7 @@ namespace Catch {
         return std::string();
     }
 
+    NonCopyable::NonCopyable() = default;
+    NonCopyable::~NonCopyable() = default;
+
 }