Zend/zend_execute.c:zend_verify_arg_type() enforces some type hints:
- wrong object class (assumes an implied type hint to IS_OBJECT)
- not an array
- not a callable
- disallowed nullness for the previous cases
- for any other type hints, raise an error
Those hints are used in the Reflection API
(currently only in __toString as there is no getter).

Zend/zend_API.c:zend_parse_arg_impl() uses a provided format string
and performs automatic convertion if possible.

The zend_verify_arg_type() function is called by the Zend VM
at some low level. As PHP wants more flexibility, it should not enforce
type hinting much.
Hence, I left the current rules, but removed the last one that raises
an error for non object, array or callable types.
As a consequence, ARG_INFOs of PHP_FE can now be more precise and
helpful.

Type hinting is an available information in ReflectionParameter,
however it is only used in __toString()!

Made that information available through the getTypeHint() function,
which returns the string representation of the type hint constant.

Turned ReflectionParameter::export() and __construct()
to use ZEND_ARG_TYPE_INFO() instead of ZEND_ARG_INFO(),
in order to register the actual type hinting.

Added a test to ensure getTypeHint() works fine with type hinted
function and returns NULL for non type hinted ones.

See request #45569, the second part.