Meta Boxes: Fix error when a meta box is rendered with a static method callback.

When `WP_DEBUG` is set, ensure that a static method callback is passed to `ReflectionMethod`, instead of `ReflectionFunction`.

Merges [43830] from the 5.0 branch to trunk.

Props DrewAPicture.
Fixes #45192.


Built from https://develop.svn.wordpress.org/trunk@44170


git-svn-id: https://core.svn.wordpress.org/trunk@44000 1a063a9b-81f0-0310-95a4-ce76da25c4cd