Add cxx-benchmark-unittests target

This patch adds the cxx-benchmark-unittests target so we can start
getting test coverage on the benchmarks, including building with
sanitizers. Because we're only looking for test-coverage, the benchmarks
run for the shortest time possible, and in parallel.

The target is excluded from all by default. It only
builds and runs the libcxx configurations of the benchmarks, and not
any versions built against the systems native standard library.

llvm-svn: 346811

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