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/sem_ir/formatter.cpp

@@ -956,6 +956,17 @@ class FormatterImpl {
     }
   }
 
+  auto FormatArg(FacetTypeId id) -> void {
+    const auto& info = sem_ir_.facet_types().Get(id);
+    out_ << "<facet-type ";
+    FormatType(info.base_facet_type_id);
+    if (info.requirement_block_id.is_valid()) {
+      // TODO: include specifics
+      out_ << "+requirements";
+    }
+    out_ << ">";
+  }
+
   auto FormatArg(IntKind k) -> void { k.Print(out_); }
 
   auto FormatArg(FloatKind k) -> void { k.Print(out_); }