[flang] Handle module subprogram with interface in same (sub)module when writing module file

There's a few (3) cases where Fortran allows two distinct symbols to have
the same name in the same scope.  Module file output copes with only two of
them.  The third involves a separate module procedure that isn't separate:
both the procedure and its declared interface appear in the same (sub)module.
Fix to ensure that the interface is included in the module file output, so
that the module file reader doesn't suffer a bogus error about a "separate
module procedure without an interface".

Differential Revision: https://reviews.llvm.org/D127784