I wasn't able to build lldb using Mingw-w64 on Windows without changing
these 3 lines. It seems like `std::atomic<bool>` wasn't being found
without `#include <atomic>` and `ceil` was defaulting to `std::ceil`
instead of `std::chrono::ceil`, but I'm not smart enough to know the
root cause. I'm sure I'm not the first people to try and compile lldb
(and clang and lld) with Mingw-w64 and I don't know if something is
wrong with my Mingw-w64, but my changes shouldn't have any affect if
they aren't needed.
(cherry picked from commit 1a32bcb4379fb90d2b764ac33b917de1431c6b16)
9ebb0abb02