Add frontend flag `-swift-concurrency={off|limited|on}`
Replace `-warn-concurrency` with a more granular option
`-swift-concurrency=`, where the developer can select one of three
different "modes":
* `off` disables `Sendable` checking for most cases. (This is the Swift
5.5 and Swift 5.6 behavior.)
* `limited` enables `Sendable` checking within code that has adopted
Swift concurrency. (This is currently the default behavior.)
* `on` enables `Sendable` and other concurrency checking throughout
the module. (This is equivalent to `-warn-concurrency` now).
There is currently no distinction between `off` and `limited`. That
will come soon.
Implements the flag part of rdar://91930849.
