Allan Shortlidge 7996c5b30c Sema: Avoid decoding unavailable enum elements in derived Codable conformances. ...

The compiler derived implementations of `Codable` conformances for enums did
not take enum element unavailability into account. This could result in
unavailable values being instantiated at runtime, leading to a general
violation of the invariant that unavailable code is unreachable at runtime.
This problem is possible because synthesized code is not type checked; had the
conformances been hand-written, they would have been rejected for referencing
unavailable declarations inside of available declarations.

This change specifically alters derivation for the following declarations:
- `Decodable.init(from:)`
- `Encodable.encode(to:)`
- `CodingKey.init(stringValue:)`

Resolves rdar://110098469

2023-07-26 22:48:25 -07:00

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