Add missing access modifiers to methods in `WP_Query`. Add magic methods for `__get()`, `__set()`, `__isset()`, `__unset()`, and `__call()`.

Add unit test for magic methods.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28523


git-svn-id: http://core.svn.wordpress.org/trunk@28349 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Classes that have `__set()` also need `__isset()` and `__unset()`.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28524


git-svn-id: http://core.svn.wordpress.org/trunk@28350 1a063a9b-81f0-0310-95a4-ce76da25c4cd