Compare ImplWitnessAccess into Self as canonical constants (#5883)

This makes all `.Self` references in a facet type canonically the same
(which will remain true iff they refer to the same `Self` type in the
future), removing the need to do more complex comparisons between them
using the EntityName, interface, and index. This allows the comparison
of types containing `.Self` references to be done correctly regardless
of where the `.Self` appears, as such type expressions will all be
canonically equal if they otherwise equal now, regardless of whether
they are written in the context where `.Self` could have seen different
`Self` facet types.

In order to retain access to constraints on a base `.Self` facet type,
in the case of applying `where` to an existing facet type, we:
- Give the base facet type as a `RequirementBaseFacetType` constraint so
that eval of `WhereExpr` can find and copy all the constraints off of
it.
- Introduce eager/early rewrite constraint resolution, which allows a
constraint to eagerly resolve access to earlier rewrite constraints
(`where .A = () and .B = .A` is eagerly transformed into `where .A = ()
and .B = ()`) before the full constraint resolution step. This allows
use of rewrite constraints in larger type expressions, such as `where .A
= () and .B = C(.A)` and `C` will know that the argument is `()`.

toolchain/check/eval_inst.cpp

@@ -333,6 +333,13 @@ auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
   return ConstantEvalResult::NewSamePhase(inst);
 }
 
+auto EvalConstantInst(Context& context,
+                      SemIR::ImplWitnessAccessSubstituted inst)
+    -> ConstantEvalResult {
+  return ConstantEvalResult::Existing(
+      context.constant_values().Get(inst.value_id));
+}
+
 auto EvalConstantInst(Context& context,
                       SemIR::ImplWitnessAssociatedConstant inst)
     -> ConstantEvalResult {