When an instruction makes an absolute reference to another instruction,
such as when `assoc_const` refers to the declaration of the associated
constant in an interface, substitution into that instruction should not
substitute into the referenced instruction.
Mark the corresponding `InstId` fields in the typed instructions as
being absolute by giving them a distinct ID type that `Subst` doesn't
substitute into. This formation of unnecessarily complicated SemIR that
could in some cases lead to a CHECK failure when printing formatted
SemIR because the same instruction ends up in multiple scopes.
Richard Smith
42bda1e38f