We don't need the unavailable default implementation of RNG.next() to be ABI.

Make it @_aEIC so that we're not stuck with it forever if we get a better solution someday.

Author: stephentyrone

stdlib/public/core/Random.swift

@@ -71,6 +71,7 @@ extension RandomNumberGenerator {
   // unsigned integer will be used, recursing infinitely and probably blowing
   // the stack.
   @available(*, unavailable)
+  @_alwaysEmitIntoClient
   public mutating func next() -> UInt64 { fatalError() }
 
   /// Returns a value from a uniform, independent distribution of binary data.