[Support] Avoid using main thread for llvm::parallelFor().

The llvm::parallelFor() uses threads created by ThreadPoolExecutor as well as main thread.
The index for the main thread matches with the index for the first thread created by ThreadPoolExecutor.
It results in that getThreadIndex returns the same value for different threads.
To avoid thread index clashing - do not use main thread for llvm::parallelFor():

parallel::TaskGroup TG;
for (; Begin + TaskSize < End; Begin += TaskSize) {
  TG.spawn([=, &Fn] {
    for (size_t I = Begin, E = Begin + TaskSize; I != E; ++I)
      Fn(I);
  });
}
for (; Begin != End; ++Begin)    <<<< executed by main thread.
  Fn(Begin);                     <<<<
return;                          <<<<

Differential Revision: https://reviews.llvm.org/D142317