[stdlib] Make unavailable protocol implementation @_alwaysEmitIntoClient and add a comment about use of Lemire's nearly divisionless method (swiftlang#38263)

Author: xwu

stdlib/public/core/Random.swift

@@ -104,6 +104,9 @@ extension RandomNumberGenerator {
     upperBound: T
   ) -> T {
     _precondition(upperBound != 0, "upperBound cannot be zero.")
+    // We use Lemire's "nearly divisionless" method for generating random
+    // integers in an interval. For a detailed explanation, see:
+    // https://arxiv.org/abs/1805.10941
     var random: T = next()
     var m = random.multipliedFullWidth(by: upperBound)
     if m.low < upperBound {