Seems that VS2019 and other C++ standard libraries include <atomic> as
part of either <string> or <vector>, but not VS2017. When #38782 landed
a new usage of std::atomic started creating compilation problems in the
VS2017 builders (eg. https://ci-external.swift.org/job/oss-swift-windows-x86_64/8736/).
Including the <atomic> header solves the problem.