Plugins: Restore backward compatibility for PHP4-style passing of `array(...

Plugins: Restore backward compatibility for PHP4-style passing of `array( &$this )` as action argument to `do_action()`.

This is a follow-up to [46149] to avoid unnecessary breakage in case of using the old notation.

Props david.binda, jrf.
Reviewed by azaozz.
Fixes #48312.
Built from https://develop.svn.wordpress.org/trunk@46568


git-svn-id: https://core.svn.wordpress.org/trunk@46365 1a063a9b-81f0-0310-95a4-ce76da25c4cd