Add support for tentative return types of internal methods

RFC: https://wiki.php.net/rfc/internal_method_return_types

Closses GH-6971

Another pass of improving ext/date argument names

Store default parameter values of internal functions in arg info

Closes GH-5353. From now on, PHP will have reflection information
about default values of parameters of internal function

Store default parameter values of internal functions in arg info

Closes GH-5353. From now on, PHP will have reflection information
about default values of parameters of internal functions.

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

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