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/eval_inst.cpp

@@ -389,7 +389,8 @@ auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
         auto index = static_cast<size_t>(inst.index.index);
         CARBON_CHECK(index < elements.size(), "Access out of bounds.");
         auto element = elements[index];
-        if (element.has_value()) {
+        if (element.has_value() &&
+            element != SemIR::InstId::ImplWitnessTablePlaceholder) {
           LoadImportRef(context, element);
           return ConstantEvalResult::Existing(GetConstantValueInSpecific(
               context.sem_ir(), witness.specific_id, element));