[flang] Extension: Allow POINTER,INTENT(IN) passed objects (#172175)
ISO Fortran now accepts a non-pointer actual argument to associate with
a dummy argument with the POINTER attribute if it is also INTENT(IN), so
long as the actual argument is a valid target for the pointer. But
passed-object dummy arguments still have a blanket prohibition against
being pointers in the ISO standard. Relax that constraint in the case of
INTENT(IN) so that passed objects can also benefit from the feature.
Fixes https://github.com/llvm/llvm-project/issues/172157.