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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM.

Maybe not worth reiterating at this point given I think everyone is on the same page, but let's make sure to wait until the libc++ maintainers have taken a look before landing.

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