Switch EvalLookupSingleImplWitness from "concrete" to "final" terminology (#6246)

A `final impl` can have a symbolic witness, but that witness is still
final. Using "final" here per discussion on
[#generics-and-templates](https://discord.com/channels/655572317891461132/941071822756143115/1428851511672312120).

I'm also changing the variant a little because `concrete_witness` was
only called when `has_concrete_value` was true, so it can be more
careful about its contract. Having a more explicit `None` also
simplifies `has_value`. I think it doesn't change the overall cost much
past that.

toolchain/check/eval_inst.cpp

@@ -295,7 +295,7 @@ auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
 
   auto result = EvalLookupSingleImplWitness(context, SemIR::LocId(inst_id),
                                             inst, self_facet_value_inst_id,
-                                            /*poison_concrete_results=*/true);
+                                            /*poison_final_results=*/true);
   if (!result.has_value()) {
     // We use NotConstant to communicate back to impl lookup that the lookup
     // failed. This can not happen for a deferred symbolic lookup in a generic
@@ -303,9 +303,9 @@ auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
     // evaluated here) to the SemIR if the lookup succeeds.
     return ConstantEvalResult::NotConstant;
   }
-  if (result.has_concrete_value()) {
+  if (result.has_final_value()) {
     return ConstantEvalResult::Existing(
-        context.constant_values().Get(result.concrete_witness()));
+        context.constant_values().Get(result.final_witness()));
   }
 
   return ConstantEvalResult::NewSamePhase(inst);