add LGTM

Author: ellishg

llvm/docs/DeveloperPolicy.rst

@@ -528,7 +528,7 @@ confirming that you have indeed had three patches committed, and that based on i
 on those reviews and elsewhere in the LLVM community they have no concern about you
 adhering to our Developer Policy and Code of Conduct. Reviewers should clearly state their
 reasoning for accepting or rejecting the request, and finish with a clear statement such
-as "I approve of this request" or "I do not approve of this request".
+as "I approve of this request", "LGTM", or "I do not approve of this request".
 
 
 If approved, a GitHub invitation will be sent to your

llvm/utils/git/github-automation.py

@@ -306,7 +306,7 @@ def run(self) -> bool:
             * Top 3 Committers: {get_user_values_str(get_top_values(merged_by))}
             * Top 3 Reviewers: {get_user_values_str(get_top_values(reviewed_by))}
 
-            Reviewers should clearly state their reasoning for accepting or rejecting this request, and finish with a clear statement such as \"I approve of this request\" or \"I do not approve of this request\". Please review the instructions for [obtaining commit access](https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access).
+            Reviewers should clearly state their reasoning for accepting or rejecting this request, and finish with a clear statement such as \"I approve of this request\", \"LGTM\", or \"I do not approve of this request\". Please review the instructions for [obtaining commit access](https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access).
         """
         self.issue.create_comment(textwrap.dedent(comment))