[OpenMP] Fold `__kmpc_is_generic_main_thread_id` if possible

This patch adds the ability to fold `__kmpc_is_generic_main_thread_id` if we
know for a fact that it is executed by the initial thread using
AAExecutionDomain. This combined with folding `__kmpc_is_spmd_exec_mode` will
allow us to fully fold `__kmpc_is_generic_main_thread`.

Depends on D106438 D106437

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106439