Update AtomicOptionalWrappable support for RawRepresentable types

[lorentey]

Also add some basic tests exercising these.

With this in place, RawRepresentable types whose RawValue is AtomicOptionalWrappable can also opt into "atomic optional wrappability" by simply conforming to the protocol -- no need to define any members:

struct Hyacinth: RawRepresentable, AtomicOptionalWrappable {
  var rawValue: UnsafeRawPointer

  init(rawValue: UnsafeRawPointer) {
    self.rawValue = rawValue
  }
}

let ref = ManagedAtomic<Hyacinth?>(nil) // OK!
ref.store(Hyacinth(rawValue: p)!, ordering: .relaxed) // OK!

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [X] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [X] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test