Support unqualified lookup into extended interfaces from classes and impls (#7217)

When a class extends an interface, referring to a member name of the
interface as an unqualified name should refer to the class's
corresponding associated entity value, not to the associated entity
itself. Similarly, in an `impl`, unqualified names of associated
entities should refer to the `impl`'s corresponding value for that
entity.

To support this, we treat `impl`s as `extend`ing their implemented facet
type, and we make lookups into an extended facet type use the `Self`
type of the extending `impl` or `class` if lookup finds an associated
entity. We already did the latter if the extending entity was an
interface; this extends the existing support for these other cases.

toolchain/check/decl_name_stack.cpp

@@ -140,6 +140,7 @@ auto DeclNameStack::AddName(NameContext name_context, SemIR::InstId target_id,
       return;
 
     case NameContext::State::Unresolved:
+    case NameContext::State::Poisoned:
       if (!name_context.parent_scope_id.has_value()) {
         AddNameToLookup(*context_, name_context.name_id, target_id,
                         name_context.initial_scope_index);