Fixes #6297: Fixes logic that checks if IntlDateFormatter parsed the string properly.

Using the `$position` as an indicator does not work properly as `IntlDateFormatter::parse()` does not return the same number as `grapheme_strlen()` for all locales (try ml-IN for an example). We don't need to know _where_ it failed, just that it _did_ fail. The `parse()` function returns `false` if it fails, so we can just check for that.

Added test for Russian, Traditional Chinese, and Malayalam.

Closes #6321