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_tostring.h

@@ -22,9 +22,10 @@
 
 #ifdef _MSC_VER
 #pragma warning(push)
-#pragma warning(disable:4180) // qualifier applied to function type has no meaning
+#pragma warning(disable:4180) // We attempt to stream a function (address) by const&, which MSVC complains about but is harmless
 #endif
 
+
 // We need a dummy global operator<< so we can bring it into Catch namespace later
 struct Catch_global_namespace_dummy;
 std::ostream& operator<<(std::ostream&, Catch_global_namespace_dummy);