[clang] [llvm] [clang][ci] Move libc++ testing into the main PR pipeline (PR #93318)

[Vlad Serebrennikov via llvm-commits]

Endilll wrote:

> You may want to have someone double-checkout those new libcxx jobs.

Sure. libc++ jobs are not really new: I basically copied them over from https://github.com/llvm/llvm-project/blob/b9d40a7ae4b3e5b9829eca8a497637c9fab6dd3e/clang/utils/ci/run-buildbot#L94-L147

https://github.com/llvm/llvm-project/pull/93318