[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

 // XRUNX: otool -L "%{lib}/libc++.1.dylib" | grep '/usr/lib/libc++.1.dylib'
 // XRUNX: ! otool -l "%{lib}/libc++.1.dylib" | grep -E "LC_RPATH|@loader_path|@rpath"
 
 // Make sure the compatibility_version of libc++ is 1.0.0.
 // Failure to respect this can result in applications not being able to find libc++
 // when they are loaded by dyld, if the compatibility version was bumped.
 //
 // RUN: otool -L "%{lib}/libc++.1.dylib" | grep "libc++.1.dylib" | grep "compatibility version 1.0.0"
+
+// Make sure we install libc++experimental.a in the right location.
+//
+// RUN: stat "%{lib}/libc++experimental.a"

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"