[C++] Support for `std::async` and `std::thread`

Are there any plans to support `std::thread` and `std::async`? I noticed that taint analysis does track through such calls. Are there any plans to do so in the future and is there a way to model around this?

The issue names no files or tests; start by locating the C++ taint-analysis modeling for std::thread and std::async. Clarify the intended support scope and modeling approach before identifying a suitable regression test and completion criteria.

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