[Bazel] Ignore both old and new benchmark directories

This is getting reverted and relanded a lot, breaking the build each
time.

Differential Revision: https://reviews.llvm.org/D114043

Author: GMNGeoffrey

utils/bazel/llvm-project-overlay/.bazelignore

@@ -2,4 +2,5 @@
 utils/bazel
 # Ignore third-party projects. These should be configured separately and some
 # include Bazel configs.
+libcxx/utils/google-benchmark
 third-party