[flang] Support external procedure passed as actual argument with implicit character type

As Fortran 2018 15.5.2.9 point 2, the actual argument and dummy argument
have the same type and type parameters and an external function with
assumed character length may be associated with a dummy argument with
explicit character length. As Fortran 2018 15.5.2.9 point 7, if an
external procedure is used as an actual argument, it can be explicitly
declared to have the EXTERNAL attribute. This supports the external
procedure passed as actual argument with implicit character type, either
explicit character length or assumed character length.

Reviewed By: Jean Perier, klausler

Differential Revision: https://reviews.llvm.org/D124345