Add CI job for clang-cl on Windows

This should catch issues like #801. This uses the new `tasks` syntax to
define multiple Windows tasks:

https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/README.md#basic-syntax

.bazelci/presubmit.yml

@@ -1,6 +1,8 @@
 ---
-platforms:
+tasks:
   ubuntu1804:
+    name: Ubuntu 18.04
+    platform: ubuntu1804
     build_flags:
     - "--features=layering_check"
     - "--copt=-Werror"
@@ -12,6 +14,8 @@ platforms:
     test_targets:
     - "//..."
   macos:
+    name: macOS: latest Xcode
+    platform: macos
     build_flags:
     - "--features=layering_check"
     - "--copt=-Werror"
@@ -22,8 +26,9 @@ platforms:
     - "--copt=-Werror"
     test_targets:
     - "//..."
-  windows:
-    # Optional: use VS 2017 instead of 2015.
+  windows-msvc:
+    name: Windows: MSVC 2017
+    platform: windows
     environment:
       BAZEL_VC: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\BuildTools\\VC"
     build_flags:
@@ -36,3 +41,22 @@ platforms:
     - "--copt=/WX"
     test_targets:
     - "//..."
+  windows-clang-cl:
+    name: Windows: Clang
+    platform: windows
+    environment:
+      BAZEL_VC: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\BuildTools\\VC"
+    build_flags:
+    - "--extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl"
+    - "--extra_execution_platforms=//:x64_windows-clang-cl"
+    - "--features=layering_check"
+    - "--copt=/WX"
+    build_targets:
+    - "//..."
+    test_flags:
+    - "--extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl"
+    - "--extra_execution_platforms=//:x64_windows-clang-cl"
+    - "--features=layering_check"
+    - "--copt=/WX"
+    test_targets:
+    - "//..."