[PATCH][Clang Driver] Driver::IsUsingLTO no longer return true when seeing -emit-llvm

[Eric Christopher]

> I think there is no path forward for supporting LTO on the commandline that
> allows all of the existing users to continue using the command lines they
> have. We should abandon that hope.
>

Cool deal. I just wanted at least one other person to say "yes, we
should do that".

> First, -O4 is a *very* bad name. LTO is not about how hard we optimize but
> about *where* we optimize. I sent out a proposal to eventually make -ON
> saturate at 3 (possibly with a warning) and there was no real arguments
> against this a long time ago. I still think that is the correct design
> there.

-O4 is an abomination. I agree.

>
> As Shuxin said, we have two *overlapping* use cases:
>
> 1) I want to do LTO.
>
> 2) I want to get the IR out for some development purpose (I do not care what
> optimization model is being requested).
>
> The only sensible flag for requesting LTO is '-flto'. Requesting LTO is the
> only sensible use of that flag.
>
> If you want IR to debug something or see the IR, you don't want to change
> the compilation or optimization model, you just want to get IR. This is
> exactly what '-emit-llvm' does and should provide.
>
> If people are surprised by the fact that '-emit-llvm' happens to be a no-op
> when using '-flto', they will learn something about or LTO.
>
> If people are surprised that passing '-flto' changes the optimization
> strategy to be better aligned with running the optimizer at link time, they
> have gotten what they asked for and need to change what they are asking for.
>

I agree with all of these things.

-eric