Isolated synchronous deinit and asynchronous deinit (#2035)

* Updated proposal

- synchronous deinit is nonisolated by default
- async deinit
- attribute to control task-local usage

* Minimized "Interaction with ObjC runtime" section

* Removed C++ code from "Runtime" section

* Removed assertion from future directions

Already implemented in swiftlang/swift@108f780

* Reviewed proposal text

* Added paragraph about task-local values and future async deinit optimizations

* Removed async deinit from the proposal

* Make task-locals undefined for isolated deinit

* Removed questionable paragraph from the Motivation section

* [SE-0371] Address some concerns about nondeterminism in motivation

* Removed few more references to async deinit

* Applied review comments

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Co-authored-by: Konrad `ktoso` Malawski <[email protected]>
Co-authored-by: Frederick Kellison-Linn <[email protected]>

Author: 3 people