[Flang] Spurious error for generic resolution, cannot disambiguate dummy between external subroutine and function

Flang incorrectly rejects valid Fortran code when resolving a generic procedure call where the actual argument is an external subroutine and one of the specific procedures has a function as the corresponding dummy argument.

Expected Behavior: External subroutine names are global identifiers. The compiler should be able to distinguish between a subroutine and a function, and correctly resolve the generic procedure call to the specific procedure that expects a subroutine dummy argument.

Actual Behavior: Flang produces the following error:
```
error: The actual arguments to the generic procedure 'gen' matched multiple specific procedures, perhaps due to use of NULL() without MOLD= or an actual procedure with an implicit interface
call gen(s, a)
```

Reproducer:

```fortran
module m
interface gen
module procedure with_sub, with_fun
end interface gen
contains
subroutine with_sub(sub,n)
external sub
call sub(n)
end subroutine with_sub
subroutine with_fun(af,n)
interface
function af(n)
real af(n)
end function
end interface
print *,'[',af(n),']'
end subroutine
end module

program test
use m
external s
integer a

a = 13
call gen(s, a)
end program

subroutine s(n)
if (n==13) print '("ok")'
end subroutine
```

Start by compiling and running the provided Fortran reproducer with Flang, then trace generic resolution for the `gen(s, a)` call and its `with_sub` and `with_fun` specifics. Done means the valid program is accepted and resolves to the specific procedure expecting the external subroutine, without the ambiguity error.

Written by the indexing model from the issue text.