[SE-0466] Disable @MainActor inference when type conforms to a SendableMetatype protocol

[DougGregor]

Libraries need a way to communicate that certain protocols can't reasonably be used with global-actor-isolated types. When the primary definition of a type conforms to such a protocol, it will disable @MainActor inference for that type. This has the effect of keeping more types nonisolated when there's signal that they should not be on the main actor. There are some protocols like this in the standard library (e.g., CodingKey, which was hardcoded to be nonisolated in the synthesis code), where the following code is ill-formed in with main-actor isolation by default:

struct S: Codable {
  var a: Int

  // error: CodingKeys inferred to `@MainActor`, which makes the conformance
  // to CodingKey main-actor-isolated and then fails (because the requirements
  // need to be nonisolated).
  enum CodingKeys: CodingKey {
    case a
  }
}

With this amendment, the conformance to CodingKey (which inherits from Sendable), prevents CodingKeys from being inferred to be@MainActor.