[flang] Clarify cases where name resolution of generic shadowing can be overridden

Fortran famously allows a generic interface definition to share a
scope with a procedure or derived type of the same name.  When that
shadowed name is accessed via host or USE association, but is also
defined by an interface in the generic, then name resolution needs
to fix up the representation of the shadowing so that the new interface
definition is seen as the shadowed symbol -- the host or USE associated
name is not material to the situation.  See the new test case for
particular examples.

Differential Revision: https://reviews.llvm.org/D136891