[-Wunreachable-code] Handle 'return' with no argument dominated by 'noreturn' function.
[CFG] Record would-be successor for noreturn destructor.
Fix CFG bug where the 'isTemporaryDtorsBranch' bit was silently lost for terminators.
[-Wunreachabe-code] Don't warn about unreachable destructors for temporaries.

This can possibly be refined later, but right now the experience
is so incomprehensible for a user to understand what is going on
this isn't a useful warning.