[flang][Semantics][OpenMP] set intrinsic attr for reductions (#85114)

Reductions such as min are intrinsic procedures. This distinguishes them
from user defined reductions. Previously, the intrinsic attribute was
not set when visiting reduction clauses causing them to be missed.

wsloop-reduction-min.f90 (the other min reduction test) worked because
it contained "min" used as an intrinsic inside of the body of the
reduction. This allowed ResolveNamesVisitor::HandleProcedureName to set
the correct attribute on that Symbol.