Reland "Add new darwin CC toolchain for tests (bazel-contrib#3460)"

This relands cd98170, which was
reverted by commit 4660427.

The Xcode toolchain is now only enabled in tests.

.bazelci/presubmit.yml

@@ -43,8 +43,16 @@ tasks:
   macos:
     shell_commands:
       - tests/core/cgo/generate_imported_dylib.sh
+    build_flags:
+      - "--apple_crosstool_top=@local_config_apple_cc//:toolchain"
+      - "--crosstool_top=@local_config_apple_cc//:toolchain"
+      - "--host_crosstool_top=@local_config_apple_cc//:toolchain"
     build_targets:
     - "//..."
+    test_flags:
+      - "--apple_crosstool_top=@local_config_apple_cc//:toolchain"
+      - "--crosstool_top=@local_config_apple_cc//:toolchain"
+      - "--host_crosstool_top=@local_config_apple_cc//:toolchain"
     test_targets:
     - "//..."
   rbe_ubuntu1604:

WORKSPACE

@@ -165,3 +165,17 @@ http_archive(
 load("@io_bazel_stardoc//:setup.bzl", "stardoc_repositories")
 
 stardoc_repositories()
+
+# For testing objc_library interop, users should not need to install it
+http_archive(
+    name = "build_bazel_apple_support",
+    sha256 = "77a121a0f5d4cd88824429464ad2bfb54bdc8a3bccdb4d31a6c846003a3f5e44",
+    url = "https://github.com/bazelbuild/apple_support/releases/download/1.4.1/apple_support.1.4.1.tar.gz",
+)
+
+load(
+    "@build_bazel_apple_support//lib:repositories.bzl",
+    "apple_support_dependencies",
+)
+
+apple_support_dependencies()