[flang] Catch obscure error in defined assignment

A subroutine that implements a defined assignment cannot have
a dummy argument for its second operand (the RHS of the assignment)
with the POINTER or ALLOCATABLE attributes, since the RHS of
an assignment is always an expression.  This problem is flagged
as a fatal error in other compilers, so let's make it fatal here
as well.

Differential Revision: https://reviews.llvm.org/D142752