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

[via llvm-commits]

https://github.com/EricWF approved this pull request.

I'm comfortable approving this for the project, since rolling it back is trivial.

We should monitor the change to ensure it can handle the load, but otherwise I think this is likely to stick.



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