[OpenMP][Clang] Allow ancestor device modifier only with reverse offloading

OpenMP Spec 5.0 [2.12.5, Restrictions]: If a device clause in which the
ancestor device-modifier appears is present on the target construct,
then a requires directive with the reverse_offload clause must be
specified.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D118887