[4.0] Allow SingleValueContainers to decode collections

[itaiferber]

What's in this pull request?
Cherry-picks #10249 for swift-4.0-branch. Addresses SR-5089.

• Explanation: SingleValueDecondingContainers in JSON and Plist previously held the assertion that attempting to decode an array or dictionary from them was a type mismatch (since those represented unkeyed and keyed containers, respectively). This assertion is no longer true, though, since encode<T : Encodable>(_:) and decode<T : Decodable>(_:) allow you to do just that.

  This lifts the assertion and adds unit tests to both implementations to ensure this works.

• Scope: Affects those using the new Codable API to encode and decode from a single value container.

• Radar: rdar://problem/32567981

• Risk: Low

• Testing: This adds new unit tests to confirm expected behavior.

[itaiferber]

@swift-ci Please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - b676e4063d9c8a3ab72386de502052e37a961633
Test requested by - @itaiferber

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - b676e4063d9c8a3ab72386de502052e37a961633
Test requested by - @itaiferber

[itaiferber]

@swift-ci Please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - b676e4063d9c8a3ab72386de502052e37a961633
Test requested by - @itaiferber

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - b676e4063d9c8a3ab72386de502052e37a961633
Test requested by - @itaiferber

[itaiferber]

@swift-ci Please test Linux

[linqingmo]

@itaiferber

func singleValueContainer() throws -> SingleValueDecodingContainer

It seems throws can be removed, since it will never throw an error. Or it is left here for some reason?

[itaiferber]

@linqingmo throws should remain for compatibility with the protocol; the protocol still should have throws for the cases where a decoder might decide to disambiguate between a "regular array/dictionary" and a keyed/unkeyed container. It also needs to be able to throw in case it's parsing data as it decodes — if the data is discovered to be corrupted, it should be able to fail there.

[linqingmo]

@itaiferber Thanks.