Genex: Reject $<TARGET_FILE:...> for object libraries (#14532)

Teach the cmGeneratorExpressionEvaluator filesystem artifact logic
to reject OBJECT_LIBRARY targets since they have no main artifact.
Without the explicit rejection evaluation falls through to an
internal CMake error message in cmTarget::GetOutputInfo.

Extend the RunCMake.GeneratorExpression test to cover these cases.
Merge topic 'object-library-no-TARGET_FILE' into next

d960589 Genex: Reject $<TARGET_FILE:...> for object libraries (#14532)