cmSyntheticTargetCache: add a struct for synthetic target caching

Source/CMakeLists.txt

@@ -426,6 +426,7 @@ add_library(
   cmStateTypes.h
   cmStringAlgorithms.cxx
   cmStringAlgorithms.h
+  cmSyntheticTargetCache.h
   cmSystemTools.cxx
   cmSystemTools.h
   cmTarget.cxx

Source/cmSyntheticTargetCache.h

@@ -0,0 +1,15 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include <map>
+#include <string>
+
+class cmGeneratorTarget;
+
+struct cmSyntheticTargetCache
+{
+  std::map<std::string, cmGeneratorTarget const*> CxxModuleTargets;
+};