Start node stack push/pop setting IdT based on ParseNodeKind. (#2985)

I think there's more we can do here, but this seemed like a good
checkpoint to make sure the path I'm going down is roughly what you
expected. There's one actual edit in if expression structure to match
the increased enforcement.

toolchain/semantics/semantics_handle_codeblock.cpp

@@ -17,7 +17,7 @@ auto SemanticsHandleCodeBlockStart(SemanticsContext& context,
 auto SemanticsHandleCodeBlock(SemanticsContext& context,
                               ParseTree::Node /*parse_node*/) -> bool {
   context.PopScope();
-  context.node_stack().PopForSoloParseNode(ParseNodeKind::CodeBlockStart);
+  context.node_stack().PopForSoloParseNode<ParseNodeKind::CodeBlockStart>();
   return true;
 }