[flang] Handle forward reference to shadowing derived type from IMPLICIT (#87280)

A derived type name in an IMPLICIT statement might be a host association
or it might be a forward reference to a local derived type, which may be
shadowing a host-associated name. Add a scan over the specification part
in search of derived type definitions to determine the right
interpretation.

Fixes https://github.com/llvm/llvm-project/issues/87215.