ICU-22901 Update ulocimp_getSubtags() &co. to use std::string_view

[roubert]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22901
• 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

ALLOW_MANY_COMMITS=true

[markusicu]

nice!