Issue Description:

If we have an immutable property with an initial value in a Codable type, then decoding the type will not set the value of the property to the decoded value. For example:

struct Foo: Codable {
  let bar: Int = 0
}

let decoder = JSONDecoder()
let foo = try! decoder.decode(Foo.self, from: Data("""
{
  "bar": 1,
}
""".utf8))
print(foo.bar) // 0

This is expected, because bar's value cannot be overwritten, however combined with the "magical" behaviour of Codable, this can be very confusing to many people.

We should offer a warning, along the lines of property 'foo' will not be decoded because it's immutable with an initial value; explicitly define a CodingKeys enum {with|without} the property, along with a fix-it to make the property mutable. This will make the behaviour more explicit to the users.

The {with|without} part depends on whether the type is only Decodable, in which case the property should not be included in the enum. If it's Encodable (or both Encodable/Decodable) then the property should be included.