The C++ interop benchmarks were only running when building with CMake, not with SwiftPM, because of a bug that was only triggering with SwiftPM. That bug seems to be fixed now.
This adds an `import CxxStdlib` statement which fixes compilation. It should be redundant, but it works around a bug that got exposed by an change in explicit modules (rdar://128520766).
This will bring back the performance numbers while the underlying issue is being investigated.