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.
Requires !8203 (merged).
Edited by Ben Boeckel