[libc++abi] Assume that libcxx is checked out in the Apple build script

Instead of using the libc++ headers provided alongside the toolchain,
use those in the sibling libcxx directory that we know is checked out.
Before the days of the monorepo, we couldn't assume that the libc++
repository was present when building libcxxabi. Since we can now make
that assumption, it's always better to use the version of libc++ that
is in lockstep with libc++abi, to avoid subtle bugs.

libcxx/utils/ci/apple-install-libcxxabi.sh

@@ -120,7 +120,7 @@ for arch in ${architectures}; do
             -DCMAKE_INSTALL_NAME_DIR="${install_name_dir}" \
             -DCMAKE_OSX_ARCHITECTURES="${arch}" \
             -DLIBCXXABI_LIBRARY_VERSION="${version}" \
-            -DLIBCXXABI_LIBCXX_INCLUDES="$(xcrun --sdk "${sdk}" c++ -print-resource-dir)/../../../include/c++/v1"
+            -DLIBCXXABI_LIBCXX_PATH="${llvm_root}/libcxx"
     )
 
     xcrun --sdk "${sdk}" cmake --build "${build_dir}/${arch}" --target install-cxxabi -- -v