For special properties like `INCLUDE_DIRECTORIES`, the pointer returned
by `cmTarget::GetProperty` is only valid until the next time the same
special property is queried on *any* target.  When evaluating a nested
`TARGET_PROPERTY` generator expression we may look up such a property
more than once on different targets.  Fix `TargetPropertyNode::Evaluate`
to store the lookup result in locally owned memory earlier.

Fixes: #19286