DNS change for CDN: apt / releases

Dear All,

I'm switching DNS entries for our CDN that serves apt.llvm.org and
releases.llvm.org. As a result we will have dual-stack support and
both APT and releases repositories will be available via IPv6 as well.
I do not expect any problems with such a change though.

On behalf of LLVM Foundation I would like to extend my gratitude to
Fastly that provides CDN for LLVM resources.