[Flang][OpenMP] Fix default firstprivatization miscategorization of mod file symbols (#157009)
In at least certain cases, notably when equivalence is used (at least
for the example this showed up as a problem in) we currently
miscategorize symbols as firstprivate when they may not be, as they can
throw a false positive when a use symbol from a mod file is picked up.
The fix to this is to chase up the appropriate symbol to access the
correct details.