Bug #19473. If you pass an allocator to std::function, we should use that allocator, not construct one from scratch. Add a test to make sure

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@206623 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.con/alloc_function.pass.cpp

@@ -92,6 +92,15 @@ int main()
     }
     assert(new_called == 0);
     {
+    assert(new_called == 0);
+    non_default_test_allocator<std::function<int(int)>> al(1);
+    std::function<int(int)> f2(std::allocator_arg, al, g);
+    assert(new_called == 0);
+    assert(f2.target<int(*)(int)>());
+    assert(f2.target<A>() == 0);
+    }
+    assert(new_called == 0);
+    {
     std::function<int(int)> f;
     assert(new_called == 0);
     assert(f.target<int(*)(int)>() == 0);