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.h

@@ -15,8 +15,6 @@
 #include <vector>
 #include <memory>
 
-CATCH_INTERNAL_SUPPRESS_ETD_WARNINGS
-
 namespace Catch {
 namespace TestCaseTracking {
 
@@ -32,7 +30,7 @@ namespace TestCaseTracking {
     using ITrackerPtr = std::shared_ptr<ITracker>;
 
     struct ITracker {
-        virtual ~ITracker() = default;
+        virtual ~ITracker();
 
         // static queries
         virtual NameAndLocation const& nameAndLocation() const = 0;
@@ -182,6 +180,4 @@ using TestCaseTracking::IndexTracker;
 
 } // namespace Catch
 
-CATCH_INTERNAL_UNSUPPRESS_ETD_WARNINGS
-
 #endif // TWOBLUECUBES_CATCH_TEST_CASE_TRACKER_HPP_INCLUDED