[stdlib] Workaround C++ lifetime rules

[davezarzycki]

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#900

[davezarzycki]

@swift-ci please smoke test

[mikeash]

LGTM. Thanks again.

[davezarzycki]

(Thinking out loud.) I wonder if we ought to just have a forEach method on the concurrent collection that internally snapshots rather than expose the snapshot method? Otherwise all of the callers need to know about the C++ lifetime wart. What do you think?

[mikeash]

There’s a way to make the methods on the snapshot not callable on a temporary. I’m going to try that out tomorrow. If that ends up being unworkable, some sort of callback-based API would be a good plan B.

[davezarzycki]

@mikeash – We can close this now because of #35345 right?

[mikeash]

Yeah, this isn't actually needed after all.