Add test for polymorphic non-exported function

tests_ispcpp/function.ispc

@@ -0,0 +1,17 @@
+floating saxpy_helper(floating scale,
+                      floating<0> x,
+                      floating<0> y) {
+    return scale * x + y;
+}
+
+
+export void saxpy(uniform int N,
+                  uniform floating<0> scale,
+                  uniform floating<1> X[],
+                  uniform floating<1> Y[],
+                  uniform floating<2> result[])
+{
+    foreach (i = 0 ... N) {
+        result[i] = saxpy_helper(scale, X[i], Y[i]);
+    }
+}