Add format checking in PR pipeline

In the same vein as commit 868fd2d7, this adds a very quick
clang-format job that checks for conformance, similar to the Travis CI
job did in commit 54273fdd (greenplum-db/gpdb#10357).

(cherry picked from commit 6aac45f3)

concourse/pipelines/pr_pipeline.yml

@@ -48,6 +48,11 @@ resources:
       secret_access_key: {{bucket-secret-access-key}}
       versioned_file: {{binary_swap_gpdb_centos_versioned_file}}
 
+  - name: clang_toolchain
+    type: registry-image
+    source:
+      repository: gcr.io/data-orca/clang-toolchain
+
 jobs:
   - name: compile-and-test-pull-request
     plan:
@@ -60,6 +65,7 @@ jobs:
           - get: centos-gpdb-dev-6
           - get: gpaddon_src
           - get: gpdb_5x_upstream
+          - get: clang_toolchain
       # We use the 5X_STABLE source for running make sync_tools instead for security.
       # For example, this would prevent a user to submit a PR that maliciously
       # modifies the sync_tools task to expose the ivy credentials.
@@ -112,6 +118,9 @@ jobs:
                   git submodule update --init --recursive
             inputs: [{ name: gpdb_pr }]
             outputs: [{ name: gpdb_src }]
+        - task: check_format
+          image: clang_toolchain
+          file: gpdb_src/concourse/tasks/check_format.yml
         - task: compile_gpdb
           file: gpdb_src/concourse/tasks/compile_gpdb.yml
           image: centos-gpdb-dev-6