[stdlib] Dictionary.merge: Don’t leave storage in an inconsistent state when closure throws

[lorentey]

The new merge implementation leaves behind an uninitialized value when the uniquing closure throws.

[lorentey]

@swift-ci please test and merge

[jrose-apple]

I think you can optimize the happy path by moving back in the failure case:

let v = (_values + bucket.offset).move()
do {
  let newValue = try combine(v, value)
  (_values + bucket.offset).initialize(to: newValue)
} catch _MergeError.keyCollision {
  fatalError("Duplicate values for key: '\(key)'")
} catch {
  // Put the value back.
  (_values + bucket.offset).initialize(to: v)
  throw error
}