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

[Aiden Grossman via llvm-commits]

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@@ -10,6 +10,7 @@ on:
   push:
     branches:
       - 'main'
+      - 'release/**'
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boomanaiden154 wrote:

Sounds good. I'd like to have everything ready by then, but can't make any guarantees. We'll probably have to backport some patches to the release branch as once we validate the new premerge workflow, we want to stop running the Buildkite pipeline, but they should be lowrisk (only impacting CI), and I can help with that.

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