[orc-rt] std::forward unbound arguments when using bind_front. (#155723)

The call operator for the type returned by bind_front should forward its
arguments, rather than moving them. This ensures that we handle lvalue
references in unbound args correctly:

  auto B = bind_front([](int &) {});
  int N = 7;
  B(N); // <- no longer an error!

Author: lhames

orc-rt/include/orc-rt/bind.h

@@ -25,7 +25,7 @@ template <typename Fn, typename... BoundArgTs> class BoundFn {
 private:
   template <size_t... Is, typename... ArgTs>
   auto callExpandingBound(std::index_sequence<Is...>, ArgTs &&...Args) {
-    return F(std::get<Is>(BoundArgs)..., std::move(Args)...);
+    return F(std::get<Is>(BoundArgs)..., std::forward<ArgTs>(Args)...);
   }
 
 public:

orc-rt/unittests/bind-test.cpp

@@ -71,6 +71,12 @@ TEST(BindTest, MinimalCopies) {
   EXPECT_EQ(OpCounter::destructions(), 2U);
 }
 
+TEST(BindTest, ForwardUnboundArgs) {
+  auto B = bind_front([](int &) {});
+  int N = 7;
+  B(N);
+}
+
 static int increment(int N) { return N + 1; }
 
 TEST(BindTest, BindFunction) {