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.
* [Concurrency] Fix templated code in Task+startSynchronously.swift
rdar://147348183
The gyb code always dropped the `throws` modifier, no matter which case was being processed.
* Update Task+startSynchronously.swift.gyb
* Fix abi tests
* [Concurrency] Initial steps for startSynchronously for Task
* [Concurrency] Rename to _startSynchronously while in development
* [Concurrency] StartSynchronously special executor to avoid switching
* startSynchronously bring back more info output
* [Concurrency] startSynchronously with more custom executor tests
* add missing ABI additions to test for x86
* [Concurrency] gyb generate _startSynchronously
* [Concurrency] %import dispatch for Linux startSynchronously test
* [Concurrency] Add TaskGroup.startTaskSynchronously funcs
* [Concurrency] DispatchSerialQueue does not exist on linux still
