Add facet type values and an instruction that produces them (#4460)

Still to do:
* Represent facet type values in a canonical form
* Produce & consume facet type values instead of interface values
* `type` should be associated with a canonical facet type value
* Support `&` on facet type values
* Type check and enforce requirements in facet types

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/handle_where.cpp

@@ -79,6 +79,7 @@ auto HandleParseNode(Context& context, Parse::RequirementEqualEqualId node_id)
   auto rhs = context.node_stack().PopExpr();
   auto lhs = context.node_stack().PopExpr();
   // TODO: type check lhs and rhs are comparable
+  // TODO: require that at least one side uses a designator
 
   // Build up the list of arguments for the `WhereExpr` inst.
   context.args_type_info_stack().AddInstId(
@@ -103,6 +104,7 @@ auto HandleParseNode(Context& context, Parse::RequirementImplsId node_id)
     context.emitter().Emit(rhs_node, ImplsOnNonFacetType);
     rhs_as_type.inst_id = SemIR::InstId::BuiltinError;
   }
+  // TODO: require that at least one side uses a designator
 
   // Build up the list of arguments for the `WhereExpr` inst.
   context.args_type_info_stack().AddInstId(