Add Self to the name scope for a class. (#3736)

Rather than just adding `Self` to the lexical scope, add it to the
class's name scope so that it is visible in later lexical scopes for the
same class -- in particular, for out-of-line definitions of members.
Also switch some tests in `check/testdata/class` over to making
idiomatic use of `Self` both inside a class and out-of-line, now that it
works more consistently.

Note that this does not permit using `Class.Self`, but only because we
don't yet support keyword names after `.` at all. If that changed, one
could use `Class.Self` to redundantly refer to `Class`. Whether we allow
that is left to a future decision.

toolchain/check/handle_class.cpp

@@ -151,8 +151,10 @@ auto HandleClassDefinitionStart(Context& context,
   context.scope_stack().Push(class_decl_id, class_info.scope_id);
 
   // Introduce `Self`.
-  context.AddNameToLookup(SemIR::NameId::SelfType,
-                          context.types().GetInstId(class_info.self_type_id));
+  context.name_scopes()
+      .Get(class_info.scope_id)
+      .names.insert({SemIR::NameId::SelfType,
+                     context.types().GetInstId(class_info.self_type_id)});
 
   context.inst_block_stack().Push();
   context.node_stack().Push(parse_node, class_id);