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_assertionresult.h

@@ -23,13 +23,12 @@ namespace Catch {
 
         AssertionResultData( ResultWas::OfType _resultType, LazyExpression const& _lazyExpression );
 
-        ResultWas::OfType resultType;
         std::string message;
-
+        mutable std::string reconstructedExpression;
         LazyExpression lazyExpression;
+        ResultWas::OfType resultType;
 
         std::string reconstructExpression() const;
-        mutable std::string reconstructedExpression;
     };
 
     class AssertionResult {