Look for final impl when accessing associated constant in facet (#5269)

While facets may come with a rewrite for an associated constant, they
are symbolic. A final impl has the ability to provide a concrete value
instead, which allows generic code to use the concrete value in place of
the associated constant's (fully qualified) name.

For instance, instead of `I.Type`, the concrete type `()` can be used if
there is an `impl final [T:! type] T as I where .Type = ()` impl.

This does not yet cache the result of the lookups.

Depends on https://github.com/carbon-language/carbon-lang/pull/5255

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_impl.cpp

@@ -364,11 +364,12 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
 
   // Process modifiers.
   // TODO: Should we somehow permit access specifiers on `impl`s?
-  // TODO: Handle `final` modifier.
   auto introducer =
       context.decl_introducer_state_stack().Pop<Lex::TokenKind::Impl>();
   LimitModifiersOnDecl(context, introducer, KeywordModifierSet::ImplDecl);
 
+  bool is_final = introducer.modifier_set.HasAnyOf(KeywordModifierSet::Final);
+
   // Finish processing the name, which should be empty, but might have
   // parameters.
   auto name_context = context.decl_name_stack().FinishImplName();
@@ -387,7 +388,8 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
                            {.self_id = self_inst_id,
                             .constraint_id = constraint_inst_id,
                             .interface = CheckConstraintIsInterface(
-                                context, impl_decl_id, constraint_inst_id)}};
+                                context, impl_decl_id, constraint_inst_id),
+                            .is_final = is_final}};
   // Add the impl declaration.
   bool invalid_redeclaration = false;
   auto lookup_bucket_ref = context.impls().GetOrAddLookupBucket(impl_info);