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/TrickyTests.cpp

@@ -381,12 +381,9 @@ TEST_CASE( "has printf" ) {
     printf( "loose text artifact\n" );
 }
 
-TEST_CASE( "assertions with commas are allowed" ) {
-}
-
 namespace {
     struct constructor_throws {
-        constructor_throws() {
+        [[noreturn]] constructor_throws() {
             throw 1;
         }
     };