[libc++] Do not set CMAKE_BUILD_WITH_INSTALL_NAME_DIR

I'm not sure what that gains us, and it creates a problem when
trying to run the tests against libc++ with a custom install name
dir (e.g. /usr/lib), since the library that we link against (in
the build tree) will advertise itself as /usr/lib/libc++.dylib,
so we end up linking against the system dylib at runtime.

Differential Revision: https://reviews.llvm.org/D105499