swift234

Add _withUnsafeGuaranteedRef to Unmanaged

This is an internal method that will go away in the future.

Get the value of the unmanaged referenced as a managed reference without
consuming an unbalanced retain of it and pass it to the closure. Asserts
that there is some other reference ('the owning reference') to the
unmanaged reference that guarantees the lifetime of the unmanaged reference
for the duration of the '_withUnsafeGuaranteedRef' call.

  var owningReference = Instance()
  ...
  withExtendedLifetime(owningReference) {
    let u = Unmanaged.passUnretained(owningReference)

    for i in 0 ..< 100 {

      // Assert that the reference in 'u' is kept alive by another storage
      // location or value that holds the same reference.
      u._withUnsafeGuaranteedRef {
        $0.doSomething()
      }

    }
  } // owningReference's lifetime (reference count > 0) is guaranteed
    // for this scope.

rdar://25129935