typechecker: support for PHP5.6 variadic args

Summary: The `...$args` syntax of https://wiki.php.net/rfc/variadics should
 supplant the `...` hh-only extension in most/all cases. However, we
 will likely support both for at least a few releases.

 The issue of whether to allow a typehint `T ...$args` to be
 interpreted as `array<T> ...$args` has been punted in the HHVM
 runtime by not allowing type-hints on variadics. As a result,
 ...$args declarations currently cannot be used in hack strict files.

 A possible extension of this work would be disallowing calls to
 func_get_arg(s) -- but that may also be appropriate for linting
 tools.

Reviewed By: @jwatzman

Differential Revision: D1410635