[OpenMP] Prevent OpenMPOpt from internalizing uncalled functions

Currently OpenMPOpt will only check if a function is a kernel before deciding not to internalize it. Any uncalled function that gets internalized will be trivially dead in the module so this is unnnecessary.

Depends on D102423

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D104890