Implements #98791 (comment)

If we have a set of values like ["ab", "c!"], we can expand it to ["ab", "Ab" "aB", "AB", "c!", "C!"] and switch to case-sensitive searching.
As long as we're making use of buckets that would otherwise have been empty, this is going to be an improvement as the prefix search loop is a bit simpler due to not needing to deal with casing. In the below benchmark, it means eliminating this step from the loop.

This optimization is a bit niche (unlikely to be applicable often), but it's really cheap to check whether it could apply, and can help cases with many non-letter characters in their prefixes.

public class IgnoreCaseToOrdinal
{
    private static readonly SearchValues<string> s_values = SearchValues.Create(["ab", "c!"], StringComparison.OrdinalIgnoreCase);
    private readonly string _text = new('\n', 1000);

    [Benchmark]
    public int IndexOfAny() => _text.AsSpan().IndexOfAny(s_values);
}