[OpenMP] Prevent Attributor from deleting functions in OpenMPOptCGSCC pass

Summary:
This patch prevents the Attributor instances made in the CGSCC pass from
deleting functions. This prevents the attributor from changing the call
graph while OpenMPOpt is working with it.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D102363