Case-insensitive comparison issue with supplementary characters.

There are issues on negating the supplementary code points in StringUTF16.compareToCIImpl method as discussed in the core-libs ml.

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On July 23, 2020 the first fix for the bug was committed. However, it
includes two simple bugs of its own. They're not much more than typos,
but they break some things nonetheless, as demonstrated by the unit
tests comprising part 2 of this contribution.

(Those two bugs: In "StringUTF16.compareToCIImpl", change statements
"cp1 -= cp1;" and "cp2 -= cp2;" to, respectively, "cp1 = -cp1;" and
"cp2 = -cp2;", and those bugs are history.)
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