Add bazel-carbon-lang to bazelignore (#755)

.bazelignore

@@ -0,0 +1,6 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# Carbon creates some non-standard bazel directories, so ignore them.
+bazel-carbon-lang