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