Merge topic 'cxxmodules-nmc-duplicate-synthetic-targets'

5261af9424 cmGeneratorTarget: store synthetic targets in its cache
e0633a9517 Tests/CXXModules: add a test importing from a `Ninja` install
150d7dbd68 Tests/CXXModules: support building a project with `Ninja`
e48e5e5506 Tests/CXXModules: document `CMake_TEST_MODULE_COMPILATION` items

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !9263