Adding noreturn and hidefromvtls attributes

[Chaiomanot]

> If your patch enforcing that a function tagged with noreturn 
> does not return (so core.stdc.stdlib.exit must be tagged with 
> noreturn)?

It isn't currently compulsory. Any noreturn function will still 
compile without the attribute. If adopted, it would of course be 
prudent to make stdlib.exit noreturn (though I haven't bothered 
with it, as I haven't used stdlib.exit yet)

> A more general comment: in few years I have seen lot of wasted 
> work done on D/DMD. So I suggest first to discuss a feature, to 
> find some kind of (partial) consensus that it's an acceptable 
> idea, and only then, try to implement it. Otherwise I think 
> time is better spent fixing bugs, implementing popular (well 
> voted) enhancement requests of Bugzilla, or even better helping 
> the review of already written patches and applying them: at the 
> moment creating more pull requests is not the top priority 
> because currently there are about 115 open pull requests for 
> DMD.

That's understandable, however I didn't add these because I 
wanted to improve D. I made them because my project called for 
them, and modifying the compiler felt cleaner than hacking the 
project.