Update lldb to use swift-5.1-branch for 5.1 config

Author: shahmishal

utils/update_checkout/update-checkout-config.json

@@ -277,7 +277,7 @@
                 "clang": "swift-5.1-branch",
                 "compiler-rt": "swift-5.1-branch",
                 "swift": "swift-5.1-branch",
-                "lldb": "stable",
+                "lldb": "swift-5.1-branch",
                 "cmark": "master",
                 "llbuild": "master",
                 "swiftpm": "master",