[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 want to do pre-commit and post-commit testing, which is what we are currently doing on the release branch.

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