[flang] Silence a bogus warning (#69799)

A recent fix (https://github.com/llvm/llvm-project/pull/66232) to
interpret a hitherto unknown name whose first appearance is as the
target of a procedure pointer initializer as an external procedure has
led to some inapproprite warning messages if the name is later defined
as an external subroutine ("X was already declared as an external").

Ensure that the EXTERNAL attribute is correctly noted as being implicit,
and that it's okay that neither the Subroutine nor Function flag has yet
been set for the implicit external.