Fix reflection of method arguments on built-ins

Many methods of built-in classes have typehints even for scalar arguments
in HHVM. This breaks ReflectionArgument since it can't find a class for
scalar arguments.

This patch changes ReflectionFunctionAbstract to not pass scalar type hints to
ReflectionArguments if scalar type hints are not enabled.

Fixes #1665.
Make ReflectionParameter::isArray support unset type info