ICU-22954 USet C++ iter samples no UnicodeString

[markusicu]

... to make the sample code work without U_SHOW_CPLUSPLUS_API as discussed in #3295 (comment)

I am also moving the non-sample code above the commented-out sample code, for clarity.

Checklist

• Required: Issue filed: ICU-22954
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[roubert]

Considering how many of these Frank has fixed just in the past few years, to correct off-by-one errors and hardcoded lengths forgotten when buffer sizes were changed, I think it's a really good idea to use UPRV_LENGTHOF or sizeof expressions even in commented-out sample code.