Synchronizing sources in:
- my_wildcmp_uca_impl() handling utf8_unicode_ci
- my_wildcmp_unicode_impl() handling utf8_general_ci

The latter has already had a fix for a similar MySQL bug in utf8_general_ci:
Bug#11754 SET NAMES utf8 followed by SELECT "A\\" LIKE "A\\" returns 0
So fix is now propagated to utf8_unicode_ci.