workflows/pr-subscriber: Handle libc++ and libc++abi labels

Author: tstellar

llvm/utils/git/github-automation.py

@@ -82,7 +82,7 @@ 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()
+        self._team_name = "pr-subscribers-{}".format(label_name.replace('+', 'x')).lower()
 
     def run(self) -> bool:
         patch = None