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_spec.cpp

@@ -15,6 +15,11 @@
 
 namespace Catch {
 
+    TestSpec::Pattern::~Pattern() = default;
+    TestSpec::NamePattern::~NamePattern() = default;
+    TestSpec::TagPattern::~TagPattern() = default;
+    TestSpec::ExcludedPattern::~ExcludedPattern() = default;
+
     TestSpec::NamePattern::NamePattern( std::string const& name )
     : m_wildcardPattern( toLower( name ), CaseSensitive::No )
     {}