Make Rand deterministic (#1910)

Also re-enable auto-updating of Rand test (disabled in #1894)

explorer/interpreter/interpreter.cpp

@@ -1213,8 +1213,10 @@ auto Interpreter::StepExp() -> ErrorOr<Success> {
           CARBON_CHECK(args.size() == 2);
           const auto& low = cast<IntValue>(*args[0]).value();
           const auto& high = cast<IntValue>(*args[1]).value();
-          std::uniform_int_distribution<> distr(low, high);
-          int r = distr(generator);
+          CARBON_CHECK(high > low);
+          // We avoid using std::uniform_int_distribution because it's not
+          // reproducible across builds/platforms.
+          int r = (generator() % (high - low)) + low;
           return todo_.FinishAction(arena_->New<IntValue>(r));
         }
         case IntrinsicExpression::Intrinsic::IntEq: {

explorer/testdata/random/random.carbon

@@ -6,7 +6,8 @@
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
 // RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
-// NOAUTOUPDATE
+// AUTOUPDATE: %{explorer} %s
+// CHECK: Nice!
 // CHECK: result: 0
 
 package ExplorerTest api;