[flang] Align same_type_as result to other compilers

Unallocated unlimited polymorphic entities do not have a dynamic type set
and do not have declared type. The standard notes that the result is
processor dependent when one of the arguments of same_type_as is in this
case. Align the result to other compiler (gfortran, nvfortran).

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D145384