Update llvm-project to be a commit hash.

Author: pschuh

utils/update_checkout/update-checkout-config.json

@@ -250,7 +250,7 @@
         "tensorflow": {
             "aliases": ["tensorflow"],
             "repos": {
-                "llvm-project": "tensorflow",
+                "llvm-project": "bd31d693aabf94c5d68480408d9a1752ed900ea4",
                 "llvm": "swift-DEVELOPMENT-SNAPSHOT-2019-10-13-a",
                 "clang": "0c249864907e436c81b75007c2f9f251d0326c29",
                 "swift": "tensorflow",