[llvm][release] Reveal download links based on uploaded assets (#167688)

For the 21.x release, download links were supposed to be revealed once
all the release builds had completed successfully. In reality, MacOS
never had a successful build so I had to hand edit the release messages.
This PR fixes this by focusing instead on what is in the release assets
after the release build step has finished (in whatever state that might
be).

1. Links are now built from a format string, with the linked files being
format arguments for that string. This is a balance between ease of
editing the format, and having the file names for use later (I tried
regex-ing file names out of the final links, which can work but is error
prone and will be hard to debug in production).

Here's an example line:
```
  <!-- LINUX_X86 * [Linux x86_64](https://github.com/llvm/llvm-project/releases/download/llvmorg-vX.Y.Z-1/LLVM-vX.Y.Z-1-Linux-X64.tar.xz) ([signature](https://github.com/llvm/llvm-project/releases/download/llvmorg-vX.Y.Z-1/LLVM-vX.Y.Z-1-Linux-X64.tar.xz.jsonl)) -->
```

2. `uncomment_download_links` now looks at the release assets to decide
whether to reveal a given link line. If all the files that line links to
are present, it will be revealed. This means we can run this at the end
of release-tasks.yml, where it will be atomic.

This process is done for all assets we would like to link to, not just
the ones currently built in GitHub. So if some become automatically
built, they will "just work". If we want to hand edit the links in,
that's still an option because the full links are in the release
message, but hidden from view.

As we are looking for `<!-- SOME_TAG`, the script can be run multiple
times on the same release and it will not try to edit anything already
revealed.

As it's possible no link lines have all their files, the text
immediately after the links has been updated to make sense when there
are no links.

Initially I was going to accumulate the statuses from the release build
matrix job, but this does not seem to be possible
(https://github.com/orgs/community/discussions/17245). Workarounds for
this often include uploading artifacts from each job, which for us, is
the same as checking the release's assets.