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/parser/parse_node_kind.h

@@ -33,6 +33,8 @@ class ParseNodeKind : public CARBON_ENUM_BASE(ParseNodeKind) {
   // Returns the number of children that the node must have, often 0. Requires
   // that has_bracket is false.
   auto child_count() const -> int32_t;
+
+  using EnumBase::Create;
 };
 
 #define CARBON_PARSE_NODE_KIND(Name) \