Make OBJECT a case sensitive constant, for HHVM compatibility and general sanity.

Support `object` explicitly, and other forms using a fallback in wpdb.

fixes #27231.

Built from https://develop.svn.wordpress.org/trunk@27377


git-svn-id: http://core.svn.wordpress.org/trunk@27226 1a063a9b-81f0-0310-95a4-ce76da25c4cd