Switch from SemIR::NodeKind::Foo to SemIR::Foo::Kind wherever possible. (#3283)

This is shorter, more closely connected to code using the typed node
types, and avoids using the ambiguous word `Node` in places referring to
typed `SemIR` nodes.

toolchain/check/declaration_name_stack.cpp

@@ -118,7 +118,7 @@ auto DeclarationNameStack::UpdateScopeIfNeeded(NameContext& name_context)
   auto resolved_node =
       context_->semantics_ir().GetNode(name_context.resolved_node_id);
   switch (resolved_node.kind()) {
-    case SemIR::NodeKind::Namespace:
+    case SemIR::Namespace::Kind:
       name_context.state = NameContext::State::Resolved;
       name_context.target_scope_id =
           resolved_node.As<SemIR::Namespace>().name_scope_id;