Refactoring in commit v3.5.0-rc1~272^2~16 (cmGeneratorTarget: Add API for
property keys, 2015-10-25) changed the Xcode generator implementation of
`XCODE_ATTRIBUTE_...` properties to use the target `GetProperty` method on each
`XCODE_ATTRIBUTE_...` property listed by `GetPropertyKeys` instead of looping
over the property entries directly. This made the lookup of property names of
the form `XCODE_ATTRIBUTE_..._LOCATION` accidentally trigger the computed
property logic for the undocumented/legacy `<CONFIG>_LOCATION` property. Of
course the computed property value is not the same as the value stored in the
`XCODE_ATTRIBUTE_..._LOCATION` property. Fix the computed property logic to
avoid triggering on `XCODE_ATTRIBUTE_...` attributes.
Closes: #16319
Brad King