PATCH: [perl #89750]: Unicode regex negated case-insensitivity

PATCH: [perl #89750]: Unicode regex negated case-insensitivity

This patch causes inverted [bracketed] character classes to not handle
multi-character folds.  The reason is that these can lead to very
counter-intuitive results (see bug discussion).

In an inverted character class, only single-char folds are now
generated.  However the fold for \xDF=>ss is hard-coded in,
and it was too much trouble sending flags to the sub-sub routine that
does this, so another check is done at the point of storing the list of
multi-char folds.  Since \xDF doesn't have a single char fold, this
works.