ICU-22001 Put translation unit local definitions in unnamed namespaces

[roubert]

Any symbol that isn't intended to have external linkage should either be declared static (legacy code) or put in an unnamed namespace.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22001
• 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