XXX reintroduce clang 5.0 and 11.0 to CI

Author: kulp

.github/workflows/bindgen.yml

@@ -18,7 +18,7 @@ jobs:
           - debian: null
             cross: null
             rust: null
-        llvm_version: ["9.0"]
+        llvm_version: ["5.0", "9.0", "11.0"]
         main_tests: [1]
         release_build: [0]
         no_default_features: [0]