[llvm-branch-commits] [libcxx] Add Testing Configuration for LLVM libc (PR #165120)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/165120

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