swift234

[sourcekitd-test] Add -async flag to request options

This enables testing asyncronous requests on platforms that support it
(currently you need blocks support to call sourcekitd_send_request).
The implementation performs all -async requests and then at the end
handles all the responses in the order they were performed (possibly
blocking until they complete).

Also add a -dont-print-request flag to avoid printing request objects;
printing is fairly slow and makes it much harder to hit timing-sensitive
races, etc.