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_impl.hpp

@@ -8,39 +8,22 @@
 #ifndef TWOBLUECUBES_CATCH_IMPL_HPP_INCLUDED
 #define TWOBLUECUBES_CATCH_IMPL_HPP_INCLUDED
 
-// Collect all the implementation files together here
-// These are the equivalent of what would usually be cpp files
-
 #ifdef __clang__
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wweak-vtables"
 #endif
 
-
-#include "internal/catch_leak_detector.h"
-
+// Keep these here for external reporters
 #include "catch_test_spec.h"
 #include "catch_test_case_tracker.h"
 
+#include "catch_leak_detector.h"
+
 // Cpp files will be included in the single-header file here
 // ~*~* CATCH_CPP_STITCH_PLACE *~*~
 
 namespace Catch {
     LeakDetector leakDetector;
-
-    // These are all here to avoid warnings about not having any out of line
-    // virtual methods
-    IResultCapture::~IResultCapture() {}
-    ITestInvoker::~ITestInvoker() {}
-    ITestCaseRegistry::~ITestCaseRegistry() {}
-    IRegistryHub::~IRegistryHub() {}
-    IMutableRegistryHub::~IMutableRegistryHub() {}
-    IExceptionTranslator::~IExceptionTranslator() {}
-    IExceptionTranslatorRegistry::~IExceptionTranslatorRegistry() {}
-    IRunner::~IRunner() {}
-    IConfig::~IConfig() {}
-
-    void Config::dummy() {}
 }
 
 #ifdef __clang__