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.h

@@ -29,7 +29,7 @@ namespace Matchers {
             std::string toString() const;
 
         protected:
-            virtual ~MatcherUntypedBase() = default;
+            virtual ~MatcherUntypedBase();
             virtual std::string describe() const = 0;
             mutable std::string m_cachedToString;
         };