[libc++][Apple] Make sure we install libc++experimental.a

[ldionne]

Match downstream

[ldionne]

CI failures are unrelated and we're on them, landing.

libcxx/test/libcxx/vendor/apple/system-install-properties.sh.cpp

 
 // This file checks various properties of the installation of libc++ when built as
 // a system library on Apple platforms.
 
 // Make sure we install the libc++ headers in the right location.
 //
 // RUN: stat "%{include}/__config"
 
-// Make sure we install libc++.1.dylib in the right location.
+// Make sure we install libc++.1.dylib and libc++experimental.a in the right location.
 //
 // RUN: stat "%{lib}/libc++.1.dylib"
+// RUN: stat "%{lib}/libc++experimental.a"
 
 // Make sure we install a symlink from libc++.dylib to libc++.1.dylib.
 //
 // RUN: stat "%{lib}/libc++.dylib"
 // RUN: readlink "%{lib}/libc++.dylib" | grep "libc++.1.dylib"
 
 // Make sure the install_name is /usr/lib.
 //

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

function universal_dylib() {
     xcrun dsymutil "${build_dir}/${dylib}" -o "${symbols_dir}/${dylib}.dSYM"
     cp "${build_dir}/${dylib}" "${symbols_dir}/${dylib}"
 }
 
 if [ "$headers_only" != true ]; then
     universal_dylib libc++.1.dylib
     universal_dylib libc++abi.dylib
     (cd "${install_dir}/usr/lib" && ln -s "libc++.1.dylib" libc++.dylib)
+
+    experimental_libs=$(for arch in ${architectures}; do echo "${build_dir}/${arch}-install/lib/libc++experimental.a"; done)
+    xcrun lipo -create ${experimental_libs} -output "${install_dir}/usr/lib/libc++experimental.a"
 fi
 
 # Install the headers by copying the headers from one of the built architectures
 # into the install directory. Headers from all architectures should be the same.
 step "Installing the libc++ and libc++abi headers to ${install_dir}/usr/include"
 any_arch=$(echo ${architectures} | cut -d ' ' -f 1)
 mkdir -p "${install_dir}/usr/include"
 ditto "${build_dir}/${any_arch}-install/include" "${install_dir}/usr/include"