[Concurrency] Properly set task create flags in TaskGroup.startTaskSynchronously (#80299)

rdar://147907609

When starting tasks synchronously on task groups, the task create flags for isSynchronousStart and isDiscardingTask were always set to `false`. With this change the former will always be `true` and the latter conditionally `true` for discarding task groups.

Author: drexin

stdlib/public/Concurrency/Task+startSynchronously.swift.gyb

@@ -170,8 +170,8 @@ extension ${GROUP_TYPE} {
       inheritContext: false,
       enqueueJob: false, // don't enqueue, we'll run it manually
       addPendingGroupTaskUnconditionally: true,
-      isDiscardingTask: false,
-      isSynchronousStart: false
+      isDiscardingTask: ${'true' if 'Discarding' in GROUP_TYPE else 'false'},
+      isSynchronousStart: true
     )
 
     // Create the task in this group.