[Concurrency] New non-copyable Continuation type (#88182)
**Explanation**:
Implementation of `Continuation` as proposed in upcoming SE proposal:
https://github.com/swiftlang/swift-evolution/pull/3246
This is a ~Copyable Continuation with much stronger safety guarantees
and no runtime overhead. Refer to proposal for details.
Initial work by @fabianfett.
**Scope**: Adds a new continuation type.
**Risk**: Low, adds new type, allows `~Copyable` into existing builtin
-- I believe this change should be safe (?)
**Testing**: Added lots of tests covering problems this aims to prevent.
**Issues**:
resolves rdar://174826360
Somewhat relevant to rdar://139975911
Co-authored-by: Fabian Fett <fabianfett@apple.com>
