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.
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