Add void casts to suppress nodiscard on linear_congruential_engine.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@332567 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/numerics/rand/rand.eng/rand.eng.lcong/assign.pass.cpp

@@ -25,7 +25,7 @@ test1()
     E e1;
     E e2;
     assert(e1 == e2);
-    e1();
+    (void)e1();
     e2 = e1;
     assert(e1 == e2);
 }