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_test_case_tracker.cpp

@@ -14,7 +14,10 @@
 #include <stdexcept>
 #include <memory>
 
-CATCH_INTERNAL_SUPPRESS_ETD_WARNINGS
+#if defined(__clang__)
+#    pragma clang diagnostic push
+#    pragma clang diagnostic ignored "-Wexit-time-destructors"
+#endif
 
 namespace Catch {
 namespace TestCaseTracking {
@@ -25,6 +28,8 @@ namespace TestCaseTracking {
     {}
 
 
+    ITracker::~ITracker() = default;
+
 
     TrackerContext& TrackerContext::instance() {
         static TrackerContext s_instance;
@@ -276,4 +281,6 @@ using TestCaseTracking::IndexTracker;
 
 } // namespace Catch
 
-CATCH_INTERNAL_UNSUPPRESS_ETD_WARNINGS
+#if defined(__clang__)
+#    pragma clang diagnostic pop
+#endif