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_stream.cpp

@@ -57,6 +57,8 @@ namespace Catch {
     ///////////////////////////////////////////////////////////////////////////
 
 
+    Catch::IStream::~IStream() = default;
+
     FileStream::FileStream( std::string const& filename ) {
         m_ofs.open( filename.c_str() );
         CATCH_ENFORCE( !m_ofs.fail(), "Unable to open file: '" << filename << "'" );