[llvm] [libc++] Switch over to the LLVM-wide premerge test runners. (PR #147794)

[via llvm-commits]

EricWF wrote:

> > Do we have documentation explaining the names/purposes of the 3 runner groups, and how libc++ developers should interact or update them?
> 
> Not yet -- where should this documentation go?

Probably under libcxx/docs. I don't have a strong preference. I'm not sure if it fits well into existing files/sections, but in that case you can add a new rst file.



> > As it stand now, upgrading the container is still a manual process and a bit of a pain. With work we can cleanup our Dockerfile to better support rebuilding without affecting the compiler/tool versions. But until then, this process seems manual, and we should ensure it's done with a regular cadence.
> 
> I was hoping to get to this refactoring by the end of the week.

Amazing! It's been a long standing wart. 

https://github.com/llvm/llvm-project/pull/147794