[OpenMP] Fix hidden helper + affinity

When KMP_AFFINITY is set, each worker thread's gtid value is used as an
index into the place list to determine the thread's placement. With hidden
helpers enabled, this gtid value is shifted down leading to unexpected
shifted thread placement. This patch restores the previous behavior by
adjusting the mask index to take the number of hidden helper threads
into account.

Hidden helper threads are given the full initial mask and do not
participate in any of the other affinity mechanisms (place partitioning,
balanced affinity). Their affinity is only printed for debug builds.

Differential Revision: https://reviews.llvm.org/D101882