Implement the shell of xdebug_is_enabled

Summary: This allows for `if(!function_exists('xdebug_is_enabled') || !xdebug_is_enabled())` style-checks to see if custom behaviour should be used instead of xdebug stuff.
Closes https://github.com/facebook/hhvm/pull/3621

Reviewed By: @sgolemon

Differential Revision: D1527920
Fix JSON_BIGINT_AS_STRING in json_decode

Summary: Closes https://github.com/facebook/hhvm/pull/3625

Reviewed By: @sgolemon

Differential Revision: D1527921