fix(iam): support adding permissions to imported roles

Now create a Policy and attach it to imported roles as well.

This will only work for imported roles in the same account. If you
need to reference roles in other accounts without trying to add
these policy statements, use an `AwsPrincipal`.

Relates to #2381, #2651, #2652, #2662.

packages/@aws-cdk/aws-iam/lib/role.ts

@@ -116,13 +116,21 @@ export class Role extends Resource implements IRole {
       public readonly roleArn = roleArn;
       public readonly roleName = Stack.of(scope).parseArn(roleArn).resourceName!;
 
-      public addToPolicy(_statement: PolicyStatement): boolean {
-        // Statement will be added to resource instead
-        return false;
+      private readonly attachedPolicies = new AttachedPolicies();
+      private defaultPolicy?: Policy;
+
+      public addToPolicy(statement: PolicyStatement): boolean {
+        if (!this.defaultPolicy) {
+          this.defaultPolicy = new Policy(this, 'Policy');
+          this.attachInlinePolicy(this.defaultPolicy);
+        }
+        this.defaultPolicy.addStatement(statement);
+        return true;
       }
 
-      public attachInlinePolicy(_policy: Policy): void {
-        // FIXME: Add warning that we're ignoring this
+      public attachInlinePolicy(policy: Policy): void {
+        this.attachedPolicies.attach(policy);
+        policy.attachToRole(this);
       }
 
       public attachManagedPolicy(_arn: string): void {

packages/@aws-cdk/aws-iam/test/test.role.ts

@@ -260,5 +260,32 @@ export = {
     test.deepEqual(importedRole.roleArn, 'arn:aws:iam::123456789012:role/S3Access');
     test.deepEqual(importedRole.roleName, 'S3Access');
     test.done();
+  },
+
+  'add policy to imported role'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+    const importedRole = Role.fromRoleArn(stack, 'ImportedRole', 'arn:aws:iam::123456789012:role/MyRole');
+
+    // WHEN
+    importedRole.addToPolicy(new PolicyStatement()
+      .addAction('s3:*')
+      .addResource('xyz'));
+
+    // THEN
+    expect(stack).to(haveResource('AWS::IAM::Policy', {
+      PolicyDocument: {
+        Statement: [
+          {
+            Action: "s3:*",
+            Effect: "Allow",
+            Resource: "xyz"
+          }
+        ],
+        Version: "2012-10-17"
+      },
+      Roles: [ "MyRole" ]
+    }));
+    test.done();
   }
 };