External Libraries: Skip deprecated file warning for PHPMailer if the API is not loaded.

In [48033] the "class-phpmailer.php" file was deprecated. In test suites, this file is loaded before WordPress in order to setup a mock PHPMailer instance. This means that for test suites that are still loading that file, they'll experience fatal errors due to the undefined "_deprecated_file" function.

This commit skips issuing the deprecated file warning if the "_deprecated_file" function is not available.

Fixes #50380.

Built from https://develop.svn.wordpress.org/trunk@48034


git-svn-id: https://core.svn.wordpress.org/trunk@47801 1a063a9b-81f0-0310-95a4-ce76da25c4cd