Code Modernization: Introduce the spread operator in `wp-includes/IXR`.

Rather than relying `func_get_args()` to retrieve arbitrary function arguments, we can now use the spread operator to assign them directly to a variable.

Props kraftbj.
See #48267, #47678.
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git-svn-id: https://core.svn.wordpress.org/trunk@47973 1a063a9b-81f0-0310-95a4-ce76da25c4cd