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/context.cpp

@@ -1060,8 +1060,8 @@ class TypeCompleter {
   template <typename InstT>
     requires(
         InstT::Kind
-            .template IsAnyOf<SemIR::AssociatedEntityType, SemIR::FunctionType,
-                              SemIR::GenericClassType,
+            .template IsAnyOf<SemIR::AssociatedEntityType, SemIR::FacetType,
+                              SemIR::FunctionType, SemIR::GenericClassType,
                               SemIR::GenericInterfaceType, SemIR::InterfaceType,
                               SemIR::UnboundElementType, SemIR::WhereExpr>())
   auto BuildValueReprForInst(SemIR::TypeId /*type_id*/, InstT /*inst*/) const