[flang] Pointers returned from functions are not definable as pointers

A reference to a pointer-valued function is a "variable" in the argot of
the Fortran standard, and can be the left-hand side of an assignment
statement or passed as a definable actual argument -- but it is not a
definable pointer, and cannot be associated with a pointer dummy argument
that is not INTENT(IN).

Differential Revision: https://reviews.llvm.org/D143827