[OpenMP] Change OpenMPOpt to check openmp metadata

The metadata added in D102361 introduces a module flag that we can check
to determine if the module was compiled with `-fopenmp` enables. We can
now check for the precense of this instead of scanning the call graph
for OpenMP runtime functions.

Depends on D102361

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D102423