Add access modifiers to methods/members in `WP_Feed_Cache`, `WP_SimplePie_File`, and `WP_Feed_Cache_Transient`.

See #27881, #22234.

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git-svn-id: http://core.svn.wordpress.org/trunk@28331 1a063a9b-81f0-0310-95a4-ce76da25c4cd