Give the BindSymbolicName for .Self in a binding pattern a FacetType type (#6036)

If it's just `TypeType` then the `BindSymbolicName` appears directly in
type positions, but if it is replaced with another facet value, then we
would need to insert a `FacetAccessType` around it. By giving it a
`FacetType` type, like other `BindSymbolicName`s we make it consistent
and avoid having to introduce extra instructions.

toolchain/check/handle_binding_pattern.cpp

@@ -227,13 +227,24 @@ 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.
   context.scope_stack().PushForSameRegion();
-  MakePeriodSelfFacetValue(context, SemIR::TypeType::TypeId);
+
+  // The `.Self` must have a type of `FacetType`, so that it gets wrapped in
+  // `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);
+
+  MakePeriodSelfFacetValue(context, type_id);
   return true;
 }