[release] Add third-party tarball to release for standalone builds

[kwk]

Summary

With the advent of https://reviews.llvm.org/D131919 and
https://github.com/llvm/llvm-project/commit/a11cd0d94ed3cabf0998a0289aead05da94c86eb
the third-party directory is required to build LLVM and other packages and in standalone
builds the third-party directory is not available from the llvm tarball anymore.

llvm/utils/release/export.sh

 #!/bin/bash
 #===-- tag.sh - Tag the LLVM release candidates ----------------------------===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#
 #
 # Create branches and release candidates for the LLVM release.
 #
 #===------------------------------------------------------------------------===#
 
 set -e
 
-projects="llvm clang cmake compiler-rt libcxx libcxxabi libclc clang-tools-extra polly lldb lld openmp libunwind flang"
+projects="llvm clang cmake compiler-rt libcxx libcxxabi libclc clang-tools-extra polly lldb lld openmp libunwind flang third-party"
 
 release=""
 rc=""
 yyyymmdd=$(date +'%Y%m%d')
 snapshot=""
 template='${PROJECT}-${RELEASE}${RC}.src.tar.xz'
 
 usage() {