[ci] Make libc++ and Clang CI scripts independent

This simply moves existing CI-related scripts for Clang outside of libc++
to make the two projects independent with respect to CI.

The reason for making this change is that libc++'s CI pipeline history
was full of Clang runs, making it a lot harder to figure out information
about libc++'s own jobs. I created a pipeline for Clang and this patch
will run pre-commit CI for Clang inside that pipeline instead.

Differential Revision: https://reviews.llvm.org/D155078