[Windows] Fix race condition between state changes

Patch by Martin Andersson (martin.andersson@evoma.se)

If the process is resumed before the state is changed to "running"
there is a possibility (when single stepping) that the debugger stops
and changes the state to "stopped" before it is first changed to
"running". This causes the process to ignore the stop event (since
the state did not change) which in turn leads the DebuggerThread to
wait indefinitely for the exception predicate in HandleExceptionEvent.

Differential Revision: https://reviews.llvm.org/D62183

llvm-svn: 366703