Almost every method in `wp_xmlrpc_server` can also return `IXR_Error` and should be labeled as such. Rehabilitate some `return` syntax.

See #30224.

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In `WP_Comment_Query`, declare `$query_vars` and `$comments` as properties.

See #30224.

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In `WP_Locale`, declare `$number_format` as a property.

See #30224.

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In `WP_Tax_Query`, declare `$primary_table` and `$primary_id_column` as properties.

See #30224.

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