Remove inst_id from the public interface of ConstantId. (#4053)

Require mapping from a `ConstantId` to an `InstId` to go through the
`ConstantValueStore`.

This is a preparatory step for an upcoming generics change where
symbolic `ConstantId`s are no longer just a thin wrapper around an
`InstId` but instead are indexes into a table with additional
information about the symbolic constant beyond its `InstId`.

toolchain/check/decl_name_stack.cpp

@@ -302,7 +302,8 @@ static auto DiagnoseQualifiedDeclInUndefinedInterfaceScope(
   auto builder = context.emitter().Build(
       loc, QualifiedDeclInUndefinedInterfaceScope,
       context.sem_ir().StringifyTypeExpr(
-          context.sem_ir().constant_values().Get(interface_inst_id).inst_id()));
+          context.sem_ir().constant_values().GetConstantInstId(
+              interface_inst_id)));
   context.NoteUndefinedInterface(interface_id, builder);
   builder.Emit();
 }