In #31457, we introduced a change that made synthesis fail if one of the stack tags was a deploy-time value. Since stack tags are assigned outside a CloudFormation context, deploy-time values cannot be evaluated, so the stack ends up with a tag like `{ Key: "my-tag", Value: "${Token[1234]}" }`, which is probably not what is intended.

Worse, those tags are automatically propagated to all resources in the stack by CloudFormation, and some may validate the tag value and find that `$` or any of the other characters are not valid tag values.

The intent was that customers would be alerted to these kinds of mistakes and apply their tags to resources, or skip stacks when applying tags to large scopes:

```ts
Tags.of(this).add('my-tag', Fn.importValue('SomeExport'), {
  excludeResourceTypes: ['aws:cdk:stack'],
});
```

The previous change was a bit drastic in its attempts.  In this one we ignore the unresolved tags and add a warning instead. That way, synthesis still succeeds.

Closes #32040.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

In aws#31457, we introduced a change that made synthesis fail if one of the stack tags was a deploy-time value. Since stack tags are assigned outside a CloudFormation context, deploy-time values cannot be evaluated, so the stack ends up with a tag like `{ Key: "my-tag", Value: "${Token[1234]}" }`, which is probably not what is intended.

Worse, those tags are automatically propagated to all resources in the stack by CloudFormation, and some may validate the tag value and find that `$` or any of the other characters are not valid tag values.

The intent was that customers would be alerted to these kinds of mistakes and apply their tags to resources, or skip stacks when applying tags to large scopes:

```ts
Tags.of(this).add('my-tag', Fn.importValue('SomeExport'), {
  excludeResourceTypes: ['aws:cdk:stack'],
});
```

The previous change was a bit drastic in its attempts.  In this one we ignore the unresolved tags and add a warning instead. That way, synthesis still succeeds.

Closes aws#32040.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*