[llvm] Remove libcxx, libcxxabi and libunwind from supported LLVM_ENABLE_PROJECTS

This is a breaking change. If you were passing one of those three runtimes
in LLVM_ENABLE_PROJECTS, you need to start passing them in LLVM_ENABLE_RUNTIMES
instead. The runtimes in LLVM_ENABLE_RUNTIMES will start being built using
the "bootstrapping build" instead, which means that they will be built
using the just-built Clang. This is usually what you wanted anyway.

If you were using LLVM_ENABLE_PROJECTS=all with the explicit goal of
building these three runtimes, you can now use LLVM_ENABLE_RUNTIMES=all
and these runtimes will be built using the bootstrapping build.

NOTE: This is a re-application of 887b8bd7 which had been reverted
      in 6b03a4fe because it broke the Sphinx documentation publishers.
      The Sphinx documentation publishers have now been moved to using
      the runtimes build, so this should not be an issue anymore.

Differential Revision: https://reviews.llvm.org/D132480