This allows programs to target older OSes while using Concurrency behind an availability check. When targeting older OSes, the symbols are weak-linked and the compiler will require the use of Concurrency features to be guarded by an availability check.
rdar://75850003
* Implementations for operators and async functions on AsyncSequence per SE-0298
* Add trailing new lines to AsyncSequence adopter files
* Remove extra // from preambles
* Transition sequence properties and sequence initializers to internal
* Rename Upstream to Base
* Rename "Failable" asynchronous sequences to "Throwing"
* Make iteration consistent with closure lifetimes and track failure via state
* Use for await syntax for async function extensions on AsyncSequence
* Work-around rethrows calculations being incorrect for conformance + closure rethrowing
* Update testing names to reflect Throwing versus Failable
* Incorperate runAsyncAndBlock into the test function for suite calls
* Remove @frozen from AsyncSequence operators
* Back out reduce family functions for now due to IRGen bug
* Additional corrections for throwing cases and fix up potential linux crasher
* Add a upstream and throwing tests and correct flatMap's throwing behavior
* Unify parameter names to count
* Add a more helpful precondition on prefix
* Add tests for throwing cases of non closure async functions on AsyncSequence
* Correct missing count transition in dropFirst
* Correct missing , in assertion
* [NFC] add commentary on optimization in dropFirst on dropFirst
* Disable collect tests for non macOS platforms for now
* Disable all async sequence tests for non macOS platforms for now
