cmExportFileGenerator: export private compile info for C++ modules

When consuming exported targets which contain C++ modules, the consuming
project must be able to recompile BMI files using the original target's
flags. This is because a module source may use some private target usage
requirement but not want to propagate it to consumers. To facilitate
this, export the private information as necessary for consumers to be
able to perform the BMI compilations.

Source/cmExportFileGenerator.h

@@ -175,6 +175,9 @@ protected:
   virtual void GenerateRequiredCMakeVersion(std::ostream& os,
                                             const char* versionString);
 
+  bool PopulateCxxModuleExportProperties(
+    cmGeneratorTarget const* gte, ImportPropertyMap& properties,
+    cmGeneratorExpression::PreprocessContext ctx, std::string& errorMessage);
   bool PopulateExportProperties(cmGeneratorTarget const* gte,
                                 ImportPropertyMap& properties,
                                 std::string& errorMessage);