[Concurrency] Enable TaskGroup/DiscardingTaskGroup in Embedded Swift

[kubamracek]

This is enabling TaskGroup and DiscardingTaskGroup to be used in Embedded Swift by:

• Using the same metadata-free approach we have for Builtin.createAsyncTask and applying it to Builtin.createTaskGroup and Builtin.createTaskGroupWithFlags. We discard the metatype argument in SILGen (we do that already for Builtin.createAsyncTask), and instead deduce it from the substitution map in IRGen.
• The runtime entry point (swift_taskGroup_initializeWithFlags) doesn't offer a convenient way to pass extra data (unlike swift_task_create). I'm avoiding adding a new entry for simplicity and instead using a bit in groupFlags that means that instead of Metadata * we receive the same format of arbitrary options that swift_task_create uses (TaskOptionRecord linked list).
• IRGen under Embedded Swift mode emits TaskOptionRecord instead of the metadata pointer, and sets the bit in groupFlags.
• Like task creation, task group creation does not support throwing in Embedded Swift yet.