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_test_registry.cpp

@@ -32,4 +32,6 @@ namespace Catch {
             getMutableRegistryHub().registerStartupException();
         }
     }
+    
+    AutoReg::~AutoReg() = default;
 }