[Support][Parallel] Add sequential mode to TaskGroup::spawn().

This patch allows to specify that some part of tasks should be
done in sequential order. It makes it possible to not use
condition operator for separating sequential tasks:

TaskGroup tg;
for () {
  if(condition)      ==>   tg.spawn([](){fn();}, condition)
    fn();
  else
    tg.spawn([](){fn();});
}

It also prevents execution on main thread. Which allows adding
checks for getThreadIndex() function discussed in D142318.

The patch also replaces std::stack with std::deque in the
ThreadPoolExecutor to have natural execution order in case
(parallel::strategy.ThreadsRequested == 1).

Differential Revision: https://reviews.llvm.org/D148728