[flang] Always diagnose incompatible function results when associating function pointers

There are some exceptional cases where the compiler permits association of
procedure pointers or dummy arguments with slightly incompatible procedure
targets, but they should not override any incompatibilty of function
result types.

(Includes a second fix to resolve the original motivating test failure, in
which a COMPLEX intrinsic function was getting its result kind divided by
two due to an implicit C++ conversion of the kind to a "*kind" parse tree
node, and those legacy "COMPLEX*size" type designators' values are twice
the type kind value.)

Differential Revision: https://reviews.llvm.org/D136964