[OpenMP] Fixed a crash when offloading to x86_64 with target nowait

PR#49334 reports a crash when offloading to x86_64 with `target nowait`,
which is caused by referencing a nullptr. The root cause of the issue is, when
pushing a hidden helper task in `__kmp_push_task`, it also maps the gtid to its
shadow gtid, which is wrong.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D97329