resource/aws_rds_cluster: Add allow_major_version_upgrade argument

Reference: #13874

Currently needs some additional work (e.g. better waiters in RDS DB Cluster or RDS DB Cluster Instance handling) so testing can pass.

Output from acceptance testing:

```
    TestAccAWSRDSCluster_AllowMajorVersionUpgrade: resource_aws_rds_cluster_test.go:160: Step 4/4 error: terraform failed: exit status 1

        stderr:

        Error: Failed to modify RDS Cluster (tf-acc-test-340394054292885352): InvalidDBClusterStateFault: Cannot modify engine version without a healthy primary instance in DB cluster: tf-acc-test-340394054292885352
        	status code: 400, request id: 2607bd75-12b6-40b5-a47b-e601680d4091

--- FAIL: TestAccAWSRDSCluster_AllowMajorVersionUpgrade (1015.90s)
```

aws/resource_aws_rds_cluster.go

@@ -44,6 +44,11 @@ func resourceAwsRDSCluster() *schema.Resource {
 				Computed: true,
 			},
 
+			"allow_major_version_upgrade": {
+				Type:     schema.TypeBool,
+				Optional: true,
+			},
+
 			"availability_zones": {
 				Type:     schema.TypeSet,
 				Elem:     &schema.Schema{Type: schema.TypeString},
@@ -982,6 +987,10 @@ func resourceAwsRDSClusterUpdate(d *schema.ResourceData, meta interface{}) error
 		DBClusterIdentifier: aws.String(d.Id()),
 	}
 
+	if v, ok := d.GetOk("allow_major_version_upgrade"); ok {
+		req.AllowMajorVersionUpgrade = aws.Bool(v.(bool))
+	}
+
 	if d.HasChange("backtrack_window") {
 		req.BacktrackWindow = aws.Int64(int64(d.Get("backtrack_window").(int)))
 		requestUpdate = true

aws/resource_aws_rds_cluster_test.go

@@ -146,6 +146,60 @@ func TestAccAWSRDSCluster_basic(t *testing.T) {
 	})
 }
 
+func TestAccAWSRDSCluster_AllowMajorVersionUpgrade(t *testing.T) {
+	var dbCluster1, dbCluster2 rds.DBCluster
+	rName := acctest.RandomWithPrefix("tf-acc-test")
+	resourceName := "aws_rds_cluster.test"
+	// If these hardcoded versions become a maintenance burden, use DescribeDBEngineVersions
+	// either by having a new data source created or implementing the testing similar
+	// to TestAccAWSDmsReplicationInstance_EngineVersion
+	engine := "aurora-postgresql"
+	engineVersion1 := "10.12"
+	engineVersion2 := "11.7"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSClusterDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSClusterConfig_AllowMajorVersionUpgrade(rName, false, engine, engineVersion1),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSClusterExists(resourceName, &dbCluster1),
+					resource.TestCheckResourceAttr(resourceName, "allow_major_version_upgrade", "false"),
+					resource.TestCheckResourceAttr(resourceName, "engine", engine),
+					resource.TestCheckResourceAttr(resourceName, "engine_version", engineVersion1),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+				ImportStateVerifyIgnore: []string{
+					"allow_major_version_upgrade",
+					"apply_immediately",
+					"cluster_identifier_prefix",
+					"master_password",
+					"skip_final_snapshot",
+				},
+			},
+			{
+				Config:      testAccAWSClusterConfig_AllowMajorVersionUpgrade(rName, false, engine, engineVersion2),
+				ExpectError: regexp.MustCompile(`InvalidParameterCombination: The AllowMajorVersionUpgrade flag must be present when upgrading to a new major version`),
+			},
+			{
+				Config: testAccAWSClusterConfig_AllowMajorVersionUpgrade(rName, true, engine, engineVersion2),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSClusterExists(resourceName, &dbCluster2),
+					resource.TestCheckResourceAttr(resourceName, "allow_major_version_upgrade", "true"),
+					resource.TestCheckResourceAttr(resourceName, "engine", engine),
+					resource.TestCheckResourceAttr(resourceName, "engine_version", engineVersion2),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSRDSCluster_AvailabilityZones(t *testing.T) {
 	var dbCluster rds.DBCluster
 	rName := acctest.RandomWithPrefix("tf-acc-test")
@@ -2251,6 +2305,30 @@ resource "aws_rds_cluster" "test" {
 `, rName)
 }
 
+func testAccAWSClusterConfig_AllowMajorVersionUpgrade(rName string, allowMajorVersionUpgrade bool, engine string, engineVersion string) string {
+	return fmt.Sprintf(`
+resource "aws_rds_cluster" "test" {
+  allow_major_version_upgrade = %[2]t
+  apply_immediately           = true
+  cluster_identifier          = %[1]q
+  engine                      = %[3]q
+  engine_version              = %[4]q
+  master_password             = "mustbeeightcharaters"
+  master_username             = "test"
+  skip_final_snapshot         = true
+}
+
+# Upgrading requires a healthy primary instance
+resource "aws_rds_cluster_instance" "test" {
+  cluster_identifier = aws_rds_cluster.test.id
+  engine             = aws_rds_cluster.test.engine
+  engine_version     = aws_rds_cluster.test.engine_version
+  identifier         = %[1]q
+  instance_class     = "db.r4.large"
+}
+`, rName, allowMajorVersionUpgrade, engine, engineVersion)
+}
+
 func testAccAWSClusterConfig_AvailabilityZones(rName string) string {
 	return fmt.Sprintf(`
 data "aws_availability_zones" "available" {

website/docs/r/rds_cluster.html.markdown

@@ -97,6 +97,7 @@ the [AWS official documentation](https://docs.aws.amazon.com/cli/latest/referenc
 
 The following arguments are supported:
 
+* `allow_major_version_upgrade` - (Optional) Enable to allow major engine version upgrades when changing engine versions. Defaults to `false`.
 * `apply_immediately` - (Optional) Specifies whether any cluster modifications are applied immediately, or during the next maintenance window. Default is `false`. See [Amazon RDS Documentation for more information.](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Overview.DBInstance.Modifying.html)
 * `availability_zones` - (Optional) A list of EC2 Availability Zones for the DB cluster storage where DB cluster instances can be created. RDS automatically assigns 3 AZs if less than 3 AZs are configured, which will show as a difference requiring resource recreation next Terraform apply. It is recommended to specify 3 AZs or use [the `lifecycle` configuration block `ignore_changes` argument](/docs/configuration/resources.html#ignore_changes) if necessary.
 * `backtrack_window` - (Optional) The target backtrack window, in seconds. Only available for `aurora` engine currently. To disable backtracking, set this value to `0`. Defaults to `0`. Must be between `0` and `259200` (72 hours)