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_stringref.cpp

@@ -5,12 +5,20 @@
  *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 
+
+#if defined(__clang__)
+#    pragma clang diagnostic push
+#    pragma clang diagnostic ignored "-Wexit-time-destructors"
+#endif
+ 
 #include "catch_stringref.h"
 
 #include <ostream>
 #include <cassert>
 #include <cstring>
 
+
+
 namespace Catch {
 
     auto getEmptyStringRef() -> StringRef {
@@ -160,3 +168,7 @@ namespace Catch {
     }
         
 } // namespace Catch
+
+#if defined(__clang__)
+#    pragma clang diagnostic pop
+#endif