[flang] Clarify edge case of host association and generic interfaces

Name resolution was mishandling cases of generic interfaces and specific procedures
(sometimes complicatd by use of the same name for each) when the specific procedure
was accessed by means of host association; only the scope of the generic interface
definition was searched for the specific procedure.  Also search enclosing scopes
in the usual way.

Differential Revision: https://reviews.llvm.org/D135213