fix(iam): policies added to immutably imported role

In the refactoring done in #5569, we introduced a bug. The
`ImmutableRole` class correctly ignored policies directly added to it,
but did not ignore policies added via `Grant.addToPrincipal()`.

That's because its `IGrantable#grantPrincipal` field was being used
as the principal to grant to, which was pointing to the wrapped
role instead of the `ImmutableRole` itself.

Fix this oversight and add a test to cement it in.

Fixes #5943.

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

@@ -22,7 +22,7 @@ import { IRole } from '../role';
 export class ImmutableRole implements IRole {
   public readonly assumeRoleAction = this.role.assumeRoleAction;
   public readonly policyFragment = this.role.policyFragment;
-  public readonly grantPrincipal = this.role.grantPrincipal;
+  public readonly grantPrincipal = this;
   public readonly roleArn = this.role.roleArn;
   public readonly roleName = this.role.roleName;
   public readonly node = this.role.node;

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

@@ -62,17 +62,39 @@ describe('ImmutableRole', () => {
   });
 
   test('ignores calls to addToPolicy', () => {
-    mutableRole.addToPolicy(new iam.PolicyStatement({
+    immutableRole.addToPolicy(new iam.PolicyStatement({
       resources: ['*'],
-      actions: ['s3:*'],
+      actions: ['iam:*'],
     }));
 
-    immutableRole.addToPolicy(new iam.PolicyStatement({
+    mutableRole.addToPolicy(new iam.PolicyStatement({
       resources: ['*'],
-      actions: ['iam:*'],
+      actions: ['s3:*'],
     }));
 
-    expect(stack).toHaveResourceLike('AWS::IAM::Policy', {
+    expect(stack).toHaveResource('AWS::IAM::Policy', {
+      "PolicyDocument": {
+        "Version": "2012-10-17",
+        "Statement": [
+          {
+            "Resource": "*",
+            "Action": "s3:*",
+            "Effect": "Allow",
+          },
+        ],
+      },
+    });
+  });
+
+  test('ignores grants', () => {
+
+    iam.Grant.addToPrincipal({
+      grantee: immutableRole,
+      actions: ['s3:*'],
+      resourceArns: ['*'],
+    });
+
+    expect(stack).not.toHaveResourceLike('AWS::IAM::Policy', {
       "PolicyDocument": {
         "Statement": [
           {