Don't substitute into the targeted instructions of an associated constant. (#4342)

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.

toolchain/sem_ir/formatter.cpp

@@ -1030,6 +1030,10 @@ class FormatterImpl {
     out_ << inst_namer_->GetNameFor(scope_, id);
   }
 
+  auto FormatName(AbsoluteInstId id) -> void {
+    FormatName(static_cast<InstId>(id));
+  }
+
   auto FormatName(SpecificId id) -> void {
     const auto& specific = sem_ir_.specifics().Get(id);
     FormatName(specific.generic_id);