[github] Move repo lockdown config into llvm-project repo

We were storing this in the .github repo and moving this into llvm-project
will allow us to delete the .github repo.

Author: tstellar

.github/lockdown.yml

@@ -0,0 +1,8 @@
+# Configuration for Repo Lockdown - https://github.com/dessant/repo-lockdown
+
+skipCreatedBefore: "2020-03-21"
+
+pulls:
+  comment: >
+    This repository does not accept pull requests.
+    Please follow http://llvm.org/docs/Contributing.html#how-to-submit-a-patch for contribution to LLVM.