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_matchers.cpp

@@ -17,6 +17,8 @@ namespace Matchers {
             return m_cachedToString;
         }
 
+        MatcherUntypedBase::~MatcherUntypedBase() = default;
+
     } // namespace Impl
 } // namespace Matchers