FormattedPlaceholder improvements

[catamorphism]

Just running CI, do not review

Checklist

• [ ] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-_____
• [ ] Required: The PR title must be prefixed with a JIRA Issue number.
• [ ] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [ ] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable