🛑Compiler support for synthesizing Hashable._hash(into:)

[lorentey]

This improves compiler support for synthesizing Hashable implementations, to make use of the resilient hashing interface introduced by #14913. It removes the default implementation of _hash(into:); the requirement is now automatically synthesized as needed.

Fully synthesized implementations of Hashable now generate a _hash(into:) implementation that does the actual work of hashing a type's components. They also add a hashValue implementation that simply instantiates a new hasher and feeds it to _hash(into:). For example, here is how Hashable conformance is derived for a simple struct:

struct Book: Hashable {
  let title: String
  let authors: [Author]
  let pageCount: Int

  @derived var hashValue: Int { 
    return _hashValue(for: self) // defined in stdlib
  }
  @derived func _hash(into hasher: inout _Hasher) {
    hasher.append(title)
    hasher.append(authors)
    hasher.append(pageCount)
  }
}

This also enables partial synthesis: when exactly one of hashValue or _hash(into:) was explicitly provided, the compiler automatically synthesizes the other requirement. This works in all cases where Hashable conformance can be implemented: in structs, enums, classes or extensions.

[lorentey]

This is currently blocked by SR-7156; in particular, vtable problems cause issues in swift-corelibs-foundation.

[lorentey]

@swift-ci smoke test

[lorentey]

@swift-ci smoke test

[DougGregor]

@swift-ci please smoke test Linux

[slavapestov]

@lorentey I didn't realize the Codable vtable bug was blocking this work. I'll take a look at it soon.

[lorentey]

This is now incorporated into and superseded by #16073.