Fix weak memory emulation to avoid generating behaviors that are forbidden under C++ 20 #4057
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RalfJung
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After discussion with Viktor Vafeiadis and Michalis Kokologiannakis, we concluded that the only thing Miri still needs to do to be sound wrt C++20 is treat SC fences as RMWs to a global location -- i.e., have a global unique clock that each RMW releases to and acquires from. This is unnecessarily strong (it rules out some behaviors that would be legal), but it is currently unclear how to do better, and we still pass all our tests for generating weak behaviors so hopefully we'll still get good test coverage for other cases.
@cbeuw does this make sense to you?
Fixes #2301