[flang] Avoid spurious error message in function result compatibility

When checking function interface compatibility for procedure pointer
assignment/initialization or actual/dummy procedure association, don't
emit a diagnositic about function result shape incompatibility unless
the interfaces differ in rank or have distinct constant extents on a
dimension.  Function results whose dimensions are determined by dummy
arguments or host-associated variables are not necessarily incompatible.

Differential Revision: https://reviews.llvm.org/D132162