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/impl.cpp

@@ -249,4 +249,10 @@ auto FillImplWitnessWithErrors(Context& context, SemIR::Impl& impl) -> void {
   }
 }
 
+auto IsImplEffectivelyFinal(Context& context, const SemIR::Impl& impl) -> bool {
+  return impl.is_final ||
+         (context.constant_values().Get(impl.self_id).is_concrete() &&
+          context.constant_values().Get(impl.constraint_id).is_concrete());
+}
+
 }  // namespace Carbon::Check