[flang] Respect function vs subroutine distinction in generic matching

When checking the specific procedures of a generic interface for a
match against a given set of actual arguments, be sure to not match
a function against a subroutine call or vice versa.  (We generally
catch and warn about attempts to declare mixed interfaces, but they
are usually conforming and can be inadvertently created when generics
are merged due to USE and host association.)

Differential Revision: https://reviews.llvm.org/D139059