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_binding_pattern.cpp

@@ -302,7 +302,8 @@ auto HandleParseNode(Context& context, Parse::VarBindingPatternId node_id)
 }
 
 auto HandleParseNode(Context& context,
-                     Parse::CompileTimeBindingPatternStartId node_id) -> bool {
+                     Parse::CompileTimeBindingPatternStartId /*node_id*/)
+    -> bool {
   // Make a scope to contain the `.Self` facet value for use in the type of the
   // compile time binding. This is popped when handling the
   // CompileTimeBindingPatternId.
@@ -312,12 +313,7 @@ auto HandleParseNode(Context& context,
   // `FacetAccessType` when used in a type position, such as in `U:! I(.Self)`.
   // This allows substitution with other facet values without requiring an
   // additional `FacetAccessType` to be inserted.
-  SemIR::FacetTypeId facet_type_id =
-      context.facet_types().Add(SemIR::FacetTypeInfo{});
-  auto const_id = EvalOrAddInst<SemIR::FacetType>(
-      context, node_id,
-      {.type_id = SemIR::TypeType::TypeId, .facet_type_id = facet_type_id});
-  auto type_id = context.types().GetTypeIdForTypeConstantId(const_id);
+  auto type_id = GetEmptyFacetType(context);
 
   MakePeriodSelfFacetValue(context, type_id);
   return true;