[libc++] Use Xcode's CMake if it's present

This resolves issues when the CMake in use on the host is too old to
configure libc++ properly, but Xcode has a sufficiently recent version.
It is technically possible for the reverse issue to happen, where the
Xcode version would be too old and the user-installed version would be
better, however in the context of our build bots, we use AppleClang on
Apple platforms, and the CMake shipped with Xcode should work with the
AppleClang shipped alongside that Xcode.

Differential Revision: https://reviews.llvm.org/D102083