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_interfaces_reporter.cpp

@@ -50,6 +50,8 @@ namespace Catch {
         }
     }
 
+     AssertionStats::~AssertionStats() = default;
+
     SectionStats::SectionStats(  SectionInfo const& _sectionInfo,
                                  Counts const& _assertions,
                                  double _durationInSeconds,
@@ -60,6 +62,8 @@ namespace Catch {
         missingAssertions( _missingAssertions )
     {}
 
+    SectionStats::~SectionStats() = default;
+
 
     TestCaseStats::TestCaseStats(  TestCaseInfo const& _testInfo,
                                    Totals const& _totals,
@@ -73,6 +77,8 @@ namespace Catch {
         aborting( _aborting )
     {}
 
+    TestCaseStats::~TestCaseStats() = default;
+
 
     TestGroupStats::TestGroupStats( GroupInfo const& _groupInfo,
                                     Totals const& _totals,
@@ -87,6 +93,8 @@ namespace Catch {
         aborting( false )
     {}
 
+    TestGroupStats::~TestGroupStats() = default;
+
     TestRunStats::TestRunStats(   TestRunInfo const& _runInfo,
                     Totals const& _totals,
                     bool _aborting )
@@ -95,9 +103,14 @@ namespace Catch {
         aborting( _aborting )
     {}
 
+    TestRunStats::~TestRunStats() = default;
+
 
     bool IStreamingReporter::isMulti() const { return false; }
 
+    IReporterFactory::~IReporterFactory() = default;
+    IReporterRegistry::~IReporterRegistry() = default;
+
     void addReporter( IStreamingReporterPtr& existingReporter, IStreamingReporterPtr&& additionalReporter ) {
 
         if( !existingReporter ) {