Add *_alias rules to @bazel_tools so that people don't have to keep writing their own.

The alias_rules.bzl hack is required because Bazel at HEAD needs to be able to be built
with released Bazel, which doesn't know these rules yet.

RELNOTES: None.
PiperOrigin-RevId: 161808672

tools/cpp/alias_rules.bzl

diff --git a/tools/cpp/alias_rules.bzl b/tools/cpp/alias_rules.bzl
new file mode 100644
index 0000000..fa2423a
--- /dev/null
+++ b/tools/cpp/alias_rules.bzl
@@ -0,0 +1,20 @@
+# Copyright 2017 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Skylark rules that stub out C++-related alias rules."""
+def cc_toolchain_alias(name):
+  if hasattr(native, "cc_toolchain_alias"):
+    native.cc_toolchain_alias(name=name)
+  else:
+    pass