cxxmodules: Fix CMP0155 NEW behavior when C++ compile features are not known

With CMP0155 NEW behavior, we scan C++ sources in targets using C++ 20,
i.e., in which the `cxx_std_20` feature is available.  However, our
check for C++ feature availability may incorrectly succeed in two cases:

* MSVC versions from VS 2013 do not model C++ standard levels,
  so we assume all features are available as a heuristic to let
  projects at least try compiling with them.

* During ABI detection the `CMAKE_CXX20_COMPILE_FEATURES` variable is not
  populated so we assume all features are available, knowing that our
  ABI detection project does not need them.

For purposes of detecting targets using C++ 20, we do not want to assume
`cxx_std_20` is available, so verify that we really know it is.