Make identifying a facet type an operation on a (self+facet type) pair (#6592)

Identifying a facet type takes both a self and facet type as a pair, and
then encode the self into the IdentifiedFacetType. This makes a
constraint that requires some _other_ type implements an interface
visible in the IdentifiedFacetType. And it will help to enable facet
types with `where T impls Z` for `T` that is not `.Self` in the future.

IdentifiedFacetTypes are now stored in a CanonicalValueStore instead of
a RelationalValueStore as they key is the combination of self and
(declared) facet type together now.

When the self-type is a facet value (has type FacetType) this is most
straightforward. But when it's a type we need to construct a FacetValue
to construct a specific for a require decl, to replace the generic
binding of the symbolic `Self`, which has type FacetType. To do so, we
make a FacetValue with an empty FacetType (equivalent to TypeType). This
prevents any looking for witnesses through the FacetType, which matches
what you can get from a type directly, requiring witnesses to come from
finding an `impl` decl.

Add additional InstNamer logic for such empty facet types so they print
as `<typename>.type.facet` if possible instead of as just `facet_value`.

toolchain/check/handle_impl.cpp

@@ -228,8 +228,8 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id)
 
   // This requires that the facet type is identified. It returns None if an
   // error was diagnosed.
-  auto specific_interface = CheckConstraintIsInterface(context, impl_decl_id,
-                                                       constraint_type_inst_id);
+  auto specific_interface = CheckConstraintIsInterface(
+      context, impl_decl_id, self_type_inst_id, constraint_type_inst_id);
 
   auto impl_id = SemIR::ImplId::None;
   {