[release] Use the Bootstrapping build for building LLVM releases

Instead of using the (now deprecated) Projects build for libcxx, libcxxabi,
libunwind and compiler-rt, this patch uses the Bootstrapping build. This
implies that Clang will be built from scratch, and then the runtimes will
be built using that just-built Clang instead of the system compiler.
This is the correct way of assembling a toolchain, since we don't want
to ship runtimes that were built with a non-Clang compiler (or a
potentially older Clang).

Differential Revision: https://reviews.llvm.org/D112748