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

projects/SelfTest/TestMain.cpp

@@ -265,4 +265,4 @@ struct AutoTestReg {
         REGISTER_TEST_CASE( manuallyRegisteredTestFunction, "ManuallyRegistered" );
     }
 };
-AutoTestReg autoTestReg;
+static AutoTestReg autoTestReg;