[llvm] Upstream libc++ buildbot restarter. (PR #93582)

[via llvm-commits]

EricWF wrote:

@tstellar Once this lands we can revoke the permissions for the r[libcxx-buildbot-restarter](https://github.com/apps/libcxx-buildbot-restarter) app.

https://github.com/llvm/llvm-project/pull/93582