[sourcekit] Fix use-after-free of block inside libsourcekitdInProc sync_request

In sourcekitdInProc, we were doing a by-ref capture of a block that
didn't live as long as the asyncronous callback. Fix it by capturing
by-copy.

Found by ASan!

Author: benlangmuir

tools/SourceKit/tools/sourcekitd/bin/InProc/sourcekitdInProc.cpp

@@ -123,7 +123,7 @@ void sourcekitd_send_request(sourcekitd_object_t req,
   sourcekitd_request_retain(req);
   receiver = Block_copy(receiver);
   WorkQueue::dispatchConcurrent([=]{
-    sourcekitd::handleRequest(req, [&](sourcekitd_response_t resp) {
+    sourcekitd::handleRequest(req, [=](sourcekitd_response_t resp) {
       // The receiver accepts ownership of the response.
       receiver(resp);
       Block_release(receiver);