Do all tests pass for release/5.9? The release/5.8 branch was abandoned late last year, so I don't think it is worth fixing.
My advice would be to just get 5.9 or 5.10 working, as the Spring release usually happens in March, and install the official AL2 build of 5.8 or 5.9 in Docker to build 5.9 or 5.10 in AL2023, as my understanding is those AL2 compilers mostly work fine as bootstrap compilers (you may need to --skip-early-swift-driver on AL2023 when building with the AL2 compiler, which is irrelevant to the build anyway).
Once you have an official release for AL2023, you can start using that to bootstrap from then on.