Merge topic 'usage-requirement-purpose-specifier'

ff1693cb3c cmGeneratorTarget: Clarify enum for specifying purpose of usage requirements
662d4c5315 cmGeneratorTarget: Clarify name for evaluating compile-only usage requirements

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !9490