Add missing access modifiers to methods in `WP` and `WP_MatchesMapRegex`. Add magic `__call()` and `__get()` methods to `WP_MatchesMapRegex` for BC.

See #27881, #22234.

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git-svn-id: http://core.svn.wordpress.org/trunk@28342 1a063a9b-81f0-0310-95a4-ce76da25c4cd