[flang] Refine procedure compatibility checking

The test for compatible function results needs to be nearly strict
equality of their types, not the usual actual/dummy type compatibility
test used in other situations.  The exceptional case is that assumed
length CHARACTER function results are compatible with explicit length
results of the same kind.  In particular, a function returning a
polymorphic pointer is not compatible with a function returning a
monomorphic pointer even of the same declared type.

Differential Revision: https://reviews.llvm.org/D145094