[llvm] workflows: Add a simple pull request subscription workflow (PR #64913)

[Fangrui Song via llvm-commits]

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@@ -65,6 +66,31 @@ def run(self) -> bool:
         return False
 
 
+class PRSubscriber:
+    @property
+    def team_name(self) -> str:
+        return self._team_name
+
+    def __init__(self, token: str, repo: str, pr_number: int, label_name : str):
+        self.repo = github.Github(token).get_repo(repo)
+        self.org = github.Github(token).get_organization(self.repo.organization.login)
+        self.pr = self.repo.get_issue(pr_number).as_pull_request()
+        self._team_name = "pr-subscribers-{}".format(label_name).lower()
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MaskRay wrote:

There is some discrepancy between the label names and the pr-subscribers- team names.

For example, `backend:X86` vs [pr-subscribers-x86](https://github.com/orgs/llvm/teams/pr-subscribers-x86), and the issue-subscriber name https://github.com/orgs/llvm/teams/issue-subscribers-backend-X86 .

That said, this is a small issue. I think we can rename pr-subscribers- team names to improve consistency.

https://github.com/llvm/llvm-project/pull/64913