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_streambuf.h

@@ -8,15 +8,13 @@
 #ifndef TWOBLUECUBES_CATCH_STREAMBUF_H_INCLUDED
 #define TWOBLUECUBES_CATCH_STREAMBUF_H_INCLUDED
 
-#include "catch_compiler_capabilities.h"
-
 #include <streambuf>
 
 namespace Catch {
 
     class StreamBufBase : public std::streambuf {
     public:
-        virtual ~StreamBufBase() = default;
+        virtual ~StreamBufBase();
     };
 }