ICU-22966 Fix -Wshadow warnings in unicode/uset.h

[th1722]

Fix -Wshadow warnings displayed when compiling ICU4C 76.1 or later.

Checklist

• Required: Issue filed: ICU-22966
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
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• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

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[roubert]

LGTM

ICU4C doesn't have any kind of style guide for how to name members vs. their initializers and common usage varies around the code base, so even though this kind of Hungarian notation used here isn't commonly used within ICU4C I guess its usage here is as good as any other way to distinguish them.