[libc++] Remove support for building through llvm-config

We've decided to move away from that by requiring that libc++ is built
as part of the monorepo a while ago. This commit removes code pertaining
to that unsupported use case and produces a clear error when the user
violates that.

In fact, building outside of the monorepo will still work as long as
LLVM_PATH is pointing to the root of the LLVM project, although that
is not officially supported.