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/check/dump.cpp

@@ -170,12 +170,6 @@ LLVM_DUMP_METHOD static auto Dump(
   return SemIR::Dump(context.sem_ir(), struct_type_fields_id);
 }
 
-LLVM_DUMP_METHOD static auto Dump(const Context& context,
-                                  SemIR::TypeBlockId type_block_id)
-    -> std::string {
-  return SemIR::Dump(context.sem_ir(), type_block_id);
-}
-
 LLVM_DUMP_METHOD static auto Dump(const Context& context, SemIR::TypeId type_id)
     -> std::string {
   return SemIR::Dump(context.sem_ir(), type_id);