Track the type as written in BaseDecl and AdaptDecl. (#4564)

Represent the type as an `InstId` rather than as a `TypeId` to preserve
how it was written and better support tracking its value in a generic.
Add accessors to `Class` to get the base and adapted type to reduce code
duplication, and add `TypeStore::GetObjectRepr` to make it easier to map
from a type to its possibly-adapted object representation type. In
passing, also move `GetIntTypeInfo` and `GetUnqualifiedType` into
`TypeStore`.

This fixes specifics of generic adapters to properly look at the
specific adapted type, and also fixes importing of adapters.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/lower/handle_aggregates.cpp

@@ -102,11 +102,8 @@ auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
                 SemIR::ClassElementAccess inst) -> void {
   // Find the class that we're performing access into.
   auto class_type_id = context.sem_ir().insts().Get(inst.base_id).type_id();
-  auto class_type =
-      context.sem_ir().types().GetAs<SemIR::ClassType>(class_type_id);
-  const auto& class_info = context.sem_ir().classes().Get(class_type.class_id);
   SemIR::TypeId object_repr_id =
-      class_info.GetObjectRepr(context.sem_ir(), class_type.specific_id);
+      context.sem_ir().types().GetObjectRepr(class_type_id);
 
   // Translate the class field access into a struct access on the object
   // representation.