[llvm][release] Add links to automatically built packages on release page

This adds links to the release packages that are automatically built
using GitHub, so that users of those platforms can find them more
easily.

The approach taken:
* "LLVM x.y.z Release" becomes the title for this links section.
* No hand built files are linked to because we can't be sure
  when or if they will appear. It's better that users check the
  full file list if they need those.
* This means no Windows links, but I've specifically mentioned
  Windows just below the links to mitigate this.
* I have tried to use the vendor names for the architectures,
  that casual users would recognise.
* Their signature file is linked as well. I expect most will ignore
  this but better to show it to remind people it exists.
* I called it "signature" as a generic term to cover the .jsonl
  and .sig files, but we're not linking to any .sig files yet.

I considered generating this using a lot of templates, but
considering the small number of links and how useful it is to
see the layout in the Python file, I prefer writing it out.

We could link to all files that *usually*, *eventually* get built,
but I'm not sure how misleading that will be for users. So I'm
proposing this conservative version for now.

Author: DavidSpickett

llvm/utils/release/github-upload-release.py

@@ -45,9 +45,24 @@ def create_release(repo, release, tag=None, name=None, message=None):
         # Note that these lines are not length limited because if we do so, GitHub
         # assumes that should be how it is laid out on the page. We want GitHub to
         # do the reflowing for us instead.
+        #
+        # In the first section, only links to automatically built files are
+        # included because we know the links will become valid.
         message = dedent(
             """\
-LLVM {release} Release
+## LLVM {release} Release
+
+Note: Immediately after a release some of these links may not be valid yet.
+
+**Linux:**
+* [x86_64](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-Linux-X64.tar.xz) ([signature](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-Linux-X64.tar.xz.jsonl))
+* [Arm64](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-Linux-ARM64.tar.xz) ([signature](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-Linux-ARM64.tar.xz.jsonl))
+
+**Mac:**
+* [Apple Silicon](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-macOS-ARM64.tar.xz) (ARM64) ([signature](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-macOS-ARM64.tar.xz.jsonl))
+* [Intel](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-macOS-X64.tar.xz) (x86-64) ([signature](https://github.com/llvm/llvm-project/releases/download/llvmorg-{release}/LLVM-{release}-macOS-X64.tar.xz.jsonl))
+
+For Windows, and any other variants of platform and architecture, check the full list of release packages at the bottom of this release page.
 
 ## Package Types