[SR-12588] JSONEncoder's sortedKeys doesn't work when Key is an enum that conforms to String

[aciidgh]

Previous ID	SR-12588
Radar	rdar://problem/62202368
Original Reporter	@aciidb0mb3r
Type	Bug

Additional Detail from JIRA

Votes	1
Component/s	Compiler, Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: f44fa76a09602e1ef66b339697c43ca2

Issue Description:

The following code produces different output on each invocation.

import Foundation

enum Key: String, Codable {
    case a, b, c
}

struct MyStruct: Encodable {
    var foo: [Key: Int]
}

let t = MyStruct(foo: [
    .a: 1,
    .b: 1,
    .c: 1,
])

let encoder = JSONEncoder()
encoder.outputFormatting = [.sortedKeys, .prettyPrinted]
let encoded = try encoder.encode(t)
print(String(data: encoded, encoding: .utf8)!)

[beccadax]

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