Reimplement _TestingInterop in C++.

[grynspan]

Normally we try to reduce our C++ content, but the complexity's much lower here since we can't use Swift's atomics properly on Darwin yet and since we don't need to juggle Swift module visibility.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.