[build-script] Stop forcing the use of libcxx

[davezarzycki]

Any C++ standard library is good enough these days.

[davezarzycki]

Let's do a full test to see if anything has a lingering dependency on libcxx being explicit.

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 2fca15f638d8edd0c307b7b4b42a0977c0e52667

[jrose-apple]

Looks unrelated. Actually, those are timeouts from turning on resilience, whoops. (@slavapestov)

[davezarzycki]

Thanks @jrose-apple for checking. Given that you asked me to run this topic by the forums, how long do you want to wait for people to speak their mind about this topic?

[jrose-apple]

I'm not sure. Maybe till the end of the week? The other possibility would be to tag specific people on the forums who work on non-Apple platforms, like the IBM folks, and see if they have any specific concerns. But it's always something we can roll back if it causes problems.

(We really need that secondary buildbot suite…)

[davezarzycki]

Until the end of the week sounds fine. Somebody who works on Windows has already said via GitHub that the MS C++ standard library works fine, so now we're up three known working libraries. If it were up to me, and given that the cost of reverting this is super low, I'd just merge this PR at this point.

[davezarzycki]

Also, I don't know who the IBM folks are.

[davezarzycki]

@swift-ci please smoke test osx

[jrose-apple]

I can't reproduce the two timeouts locally (though admittedly with my own preset, not the builder's). Maybe a clean test'll clear it up?

@swift-ci Please clean smoke test macOS

[jrose-apple]

@swift-ci Please clean smoke test OS X

[davezarzycki]

@swift-ci please clean smoke test OS X

[davezarzycki]

Maybe something is out of sync on the macOS side. I'm rebasing and trying again.

@swift-ci please smoke test

[davezarzycki]

The PowerPC and S390 folks at IBM signed off on this via the forums. I'm going to merge this now. We can always revert this.