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

[Tom Stellard via llvm-commits]

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

This seems OK to me at first glance.  You may want to have someone double-checkout those new libcxx jobs.  I'm not really familiar with those.

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