make sure a computed list is can be RequiresNew

If a schema.TypeList had a Schema with ForceNew, and if that list was
NewComputed, the diff would not have RequiresNew set. This causes apply
to fail when the diffs didn't match because of the change to
RequiresNew.

Set the RequiresNew field on the list's ResourceAttrDiff based on the
Schema value.

helper/schema/schema.go

@@ -744,6 +744,7 @@ func (m schemaMap) diffList(
 		diff.Attributes[k+".#"] = &terraform.ResourceAttrDiff{
 			Old:         oldStr,
 			NewComputed: true,
+			RequiresNew: schema.ForceNew,
 		}
 		return nil
 	}

helper/schema/schema_test.go

@@ -2777,6 +2777,52 @@ func TestSchemaMap_Diff(t *testing.T) {
 				},
 			},
 		},
+
+		{
+			Name: "List with computed schema and ForceNew",
+			Schema: map[string]*Schema{
+				"config": &Schema{
+					Type:     TypeList,
+					Optional: true,
+					ForceNew: true,
+					Elem: &Schema{
+						Type: TypeString,
+					},
+				},
+			},
+
+			State: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"config.#": "2",
+					"config.0": "a",
+					"config.1": "b",
+				},
+			},
+
+			Config: map[string]interface{}{
+				"config": []interface{}{"${var.a}", "${var.b}"},
+			},
+
+			ConfigVariables: map[string]ast.Variable{
+				"var.a": interfaceToVariableSwallowError(
+					config.UnknownVariableValue),
+				"var.b": interfaceToVariableSwallowError(
+					config.UnknownVariableValue),
+			},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"config.#": &terraform.ResourceAttrDiff{
+						Old:         "2",
+						New:         "",
+						RequiresNew: true,
+						NewComputed: true,
+					},
+				},
+			},
+
+			Err: false,
+		},
 	}
 
 	for i, tc := range cases {