Remove all remaining uses of TypeIds as instruction operands. (#5280)

In preparation for shifting from `TypeId`s potentially representing
attached types to always representing unattached types, using
[terminology suggested on
Discord](https://discord.com/channels/655572317891461132/963846118964350976/1359286326779973712).
This change causes us to track slightly more type spelling information
through SemIR.

One change that has significant impact on the SemIR output is that we
now build a `struct_type` instruction in each class representing the
types of the fields, including the spelling used for those types. This
is now no longer always identical to the corresponding canonical
`struct_type` for the object representation, so it's built separately
and owned by the class.

Also remove `TypeBlock` support entirely, as its only use was
representing `TupleType`s, which now use an `InstBlock`.

toolchain/sem_ir/class.cpp

@@ -44,9 +44,11 @@ auto Class::GetObjectRepr(const File& file, SpecificId specific_id) const
   if (witness_id == ErrorInst::SingletonConstantId) {
     return ErrorInst::SingletonTypeId;
   }
-  return file.insts()
-      .GetAs<CompleteTypeWitness>(file.constant_values().GetInstId(witness_id))
-      .object_repr_id;
+  return file.types().GetTypeIdForTypeInstId(
+      file.insts()
+          .GetAs<CompleteTypeWitness>(
+              file.constant_values().GetInstId(witness_id))
+          .object_repr_type_inst_id);
 }
 
 }  // namespace Carbon::SemIR