Introduce verification checks on subtree_size (#2414)

This switches the Verify method to walk postorder so that we can see how much subtree_size is really used, and shift towards removing it. It also starts calling Verify.

Also, I think I'd lost the reserve/size check during Parser refactoring, so I'm putting that back in as part of Verify.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/parser/parse_node_kind.h

@@ -33,6 +33,9 @@ class ParseNodeKind {
   };
 
  public:
+  // TODO: See parse_node_kind.def.
+  static constexpr int32_t TodoFixParseNode = -2;
+
   // `clang-format` has a bug with spacing around `->` returns in macros. See
   // https://bugs.llvm.org/show_bug.cgi?id=48320 for details.
 #define CARBON_PARSE_NODE_KIND(Name)            \
@@ -64,6 +67,17 @@ class ParseNodeKind {
 
   void Print(llvm::raw_ostream& out) const { out << name(); }
 
+  // Returns true if the node is bracketed; otherwise, child_count is used.
+  auto has_bracket() const -> bool;
+
+  // Returns the bracketing node kind for the current node kind. Requires that
+  // has_bracket is true.
+  auto bracket() const -> 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;
+
  private:
   constexpr explicit ParseNodeKind(KindEnum k) : kind_(k) {}