Move None to IdBase (#5030)

Use CRTP to eliminate per-type declarations of `None`. Note this adds
`None` to a few that may not need it, but eliminates a lot off
boilerplate.

Note this leaves `GenericInstIndex::None` because it has a more complex
construction.

Also fix `InstId::InitTombstone` to be `NoneIndex - 1`

toolchain/parse/tree.h

@@ -29,14 +29,9 @@ struct DeferredDefinitionIndex : public IndexBase<DeferredDefinitionIndex> {
   static constexpr llvm::StringLiteral Label = "deferred_def";
   using ValueType = DeferredDefinition;
 
-  static const DeferredDefinitionIndex None;
-
   using IndexBase::IndexBase;
 };
 
-constexpr DeferredDefinitionIndex DeferredDefinitionIndex::None =
-    DeferredDefinitionIndex(NoneIndex);
-
 // A function whose definition is deferred because it is defined inline in a
 // class or similar scope.
 //