[stdlib] Make sure _SwiftNewtypeWrapper hashes the same way as its RawValue

_SwiftNewtypeWrapper forwarded hashValue to its rawValue, but it failed to do the same for _hash(into:), which resulted in the wrapper struct having subtly different hashing than the raw value.

This violated an implicit invariant when dictionaries/sets of such types were bridged to Objective-C through AnyHashable, leading to nondeterministic but frequent crashes.

rdar://problem/39398060

Author: lorentey

stdlib/public/core/NewtypeWrapper.swift

@@ -15,11 +15,16 @@
 /// attribute.
 public protocol _SwiftNewtypeWrapper : RawRepresentable { }
 
-extension _SwiftNewtypeWrapper where Self.RawValue : Hashable {
+extension _SwiftNewtypeWrapper where Self: Hashable, Self.RawValue : Hashable {
   @inlinable // FIXME(sil-serialize-all)
   public var hashValue: Int {
     return rawValue.hashValue
   }
+
+  @inlinable // FIXME(sil-serialize-all)
+  public func _hash(into hasher: inout _Hasher) {
+    hasher.append(rawValue)
+  }
 }
 
 #if _runtime(_ObjC)