Historically, we've evaluated the plural forms for each language using `create_function()`. This is being deprecated in PHP 7.2, so needs to be replaced.
The `Plural_Forms` class parses the `Plural-Forms` header from the PO file, and internally caches the result of all subsequent plural form tests, allowing it to match the performance of the existing code.
Props rmccue.
Fixes#41562.
Built from https://develop.svn.wordpress.org/trunk@41722
git-svn-id: http://core.svn.wordpress.org/trunk@41556 1a063a9b-81f0-0310-95a4-ce76da25c4cd