cmCxxModuleUsageEffects: add a class to capture module usage effects

When importing a C++ module, there may be requirements imposed by the
importer so that the compiler can reliably read the BMI. For example,
the standard used in the importer may need to also apply to the imported
BMI.

Right now, there are no tracked requirements. As we learn more, this
class can start tracking more information.

See: https://wg21.link/p2581r2