Use typed parse node ids in SemIR instruction types (#3560)

This involves a number of supporting changes:
* The `parse_node;` member of instruction types may now have any type
derived from `Parse::NodeId` and is no longer required to have that
exact type.
* `Parse::Node::Invalid` is now a singleton object of a separate type
that is convertible to `Parse::NodeId` and its descendants. This
replaces the `Invalid` member of its descendants, and avoids having to
write long `NodeIdOneOf<...>` types when initializing variables to
invalid.
* `IndexBase` now allows `==` and `!=` comparisons between its derived
classes and types that are convertible to those types.
* A number of functions in the check stage have been changed to preserve
more type information instead of using `Parse::NodeId`.
* `NodeIdForKind<K>` (also known as `KId`) now has a `Kind` member so it
may be used to declare `NodeIdOneOf<T, U>` types without #including
`parse/typed_nodes.h`.
* `NodeIdForKind<K>` (also known as `KId`) may be implicitly converted
to `NodeIdOneOf<T, U>` if `T::Kind == K` or `U::Kind == K` (executing a
TODO).

Many of the `parse_node` members were not converted since they would
have required more extensive changes. They have been marked with "TODO"
comments.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_class.cpp

@@ -21,8 +21,10 @@ auto HandleClassIntroducer(Context& context,
   return true;
 }
 
-static auto BuildClassDecl(Context& context, Parse::NodeId parse_node)
-    -> std::tuple<SemIR::ClassId, SemIR::InstId> {
+static auto BuildClassDecl(
+    Context& context,
+    Parse::NodeIdOneOf<Parse::ClassDeclId, Parse::ClassDefinitionStartId>
+        parse_node) -> std::tuple<SemIR::ClassId, SemIR::InstId> {
   auto name_context = context.decl_name_stack().FinishName();
   context.node_stack()
       .PopAndDiscardSoloParseNode<Parse::NodeKind::ClassIntroducer>();