Foundation: Provide a Hashable.hashValue witness for _BridgedStoredNSError conformers

Previously we would synthesize this in Sema, but this no longer
works when conformance checking is triggered by SILGen.

stdlib/public/Darwin/Foundation/NSError.swift

@@ -489,6 +489,10 @@ extension _BridgedStoredNSError {
   public func hash(into hasher: inout Hasher) {
     hasher.combine(_nsError)
   }
+
+  @_alwaysEmitIntoClient public var hashValue: Int {
+    return _nsError.hashValue
+  }
 }
 
 /// Describes the code of an error.