Deleted Thread.hpp and replaced AtomicInt with std::atomic<int>.
Should be noop.

Note: AtomicInt's operator++(int) and operator--(int) were
      Post-increment operators implemented as Pre-increment
      operators, so expressions <AtomicInt object>++ have
      been replaced with ++<std::atomic object> (same for
      decrement operators).

Bug b/129403963

Change-Id: I907b8c91581fed4b76647327f316b87369c6c82f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33668Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>