Refactor NodeCategory for X-macros (#5029)

Taking an approach similar to NameId in #5018

toolchain/parse/node_category.cpp

@@ -0,0 +1,37 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/parse/node_category.h"
+
+#include "llvm/ADT/StringExtras.h"
+
+namespace Carbon::Parse {
+
+// Returns a string form of the node category for printing.
+static auto NodeCategoryToString(NodeCategory::RawEnumType category)
+    -> llvm::StringLiteral {
+#define CARBON_NODE_CATEGORY_TO_STRING(Name) \
+  case NodeCategory::Name:                   \
+    return #Name;
+
+  switch (category) {
+    CARBON_NODE_CATEGORY(CARBON_NODE_CATEGORY_TO_STRING)
+    CARBON_NODE_CATEGORY_TO_STRING(None)
+  }
+
+#undef CARBON_NODE_CATEGORY_TO_STRING
+}
+
+auto NodeCategory::Print(llvm::raw_ostream& out) const -> void {
+  llvm::ListSeparator sep("|");
+  auto value = value_;
+  do {
+    // The lowest set bit in the value, or 0 (`None`) if no bits are set.
+    auto lowest_bit = static_cast<RawEnumType>(value & -value);
+    out << sep << NodeCategoryToString(lowest_bit);
+    value &= ~lowest_bit;
+  } while (value);
+}
+
+}  // namespace Carbon::Parse