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/builtin_function_kind.cpp

@@ -72,7 +72,7 @@ struct NoReturn {
     if (!tuple) {
       return false;
     }
-    return sem_ir.type_blocks().Get(tuple->elements_id).empty();
+    return sem_ir.inst_blocks().Get(tuple->elements_id).empty();
   }
 };