[libcxx] Add Testing Configuration for LLVM libc

This patch adds a CMake cache, and the testing configs to build libc++
and run the tests against LLVM libc.

Reviewers: ldionne, philnik777, jtstogel

Pull Request: https://github.com/llvm/llvm-project/pull/165120

NOKEYCHECK=True
GitOrigin-RevId: f650fba569c41637bf7bec8ed0e89bbb13bf3812