Update the llvm-project branch to stable/20211026

Author: shahmishal

utils/update_checkout/update-checkout-config.json

@@ -85,9 +85,9 @@
     "default-branch-scheme": "main",
     "branch-schemes": {
        "main": {
-            "aliases": ["swift/main", "main", "stable/20210726"],
+            "aliases": ["swift/main", "main", "stable/20211026"],
             "repos": {
-                "llvm-project": "stable/20210726",
+                "llvm-project": "stable/20211026",
                 "swift": "main",
                 "cmark": "main",
                 "llbuild": "main",