[llvm] workflows/premerge: Add macOS testing for release branch (PR #124303)

[Tom Stellard via llvm-commits]

================
@@ -10,6 +10,7 @@ on:
   push:
     branches:
       - 'main'
+      - 'release/**'
----------------
tstellar wrote:

I would like to have the mac testing ready by the LLVM 20 branch date.  For Windows and Linux we are already covered on the release branch by buildkite, so there is no rush to start using the new GitHub Actions workflow for those platforms.

If the new workflow isn't ready.  I can patch the workflow file in the release branch to drop the:

```
   paths:
      - .github/workflows/premerge.yaml
```
And also disable the linux job.

https://github.com/llvm/llvm-project/pull/124303