1. Adding a separate MY_COLLATION_HANDLER
   my_collation_ucs2_general_mysql500_ci_handler
   implementing a proper order for ucs2_general_mysql500_ci
   The problem happened because ucs2_general_mysql500_ci
   erroneously used my_collation_ucs2_general_ci_handler.

2. Cosmetic changes: Renaming:
   - plane00_mysql500 to my_unicase_mysql500_page00
   - my_unicase_pages_mysql500 to my_unicase_mysql500_pages
   to use the same naming style with:
   - my_unicase_default_page00
   - my_unicase_defaul_pages

3. Moving code fragments from
   - handler::check_collation_compatibility() in handler.cc
   - upgrade_collation() in table.cc
   into new methods in class Charset, to reuse the code easier.

This patch is the result of running
run-clang-tidy -fix -header-filter=.* -checks='-*,modernize-use-equals-default' .

Code style changes have been done on top. The result of this change
leads to the following improvements:

1. Binary size reduction.
* For a -DBUILD_CONFIG=mysql_release build, the binary size is reduced by
  ~400kb.
* A raw -DCMAKE_BUILD_TYPE=Release reduces the binary size by ~1.4kb.

2. Compiler can better understand the intent of the code, thus it leads
   to more optimization possibilities. Additionally it enabled detecting
   unused variables that had an empty default constructor but not marked
   so explicitly.

   Particular change required following this patch in sql/opt_range.cc

   result_keys, an unused template class Bitmap now correctly issues
   unused variable warnings.

   Setting Bitmap template class constructor to default allows the compiler
   to identify that there are no side-effects when instantiating the class.
   Previously the compiler could not issue the warning as it assumed Bitmap
   class (being a template) would not be performing a NO-OP for its default
   constructor. This prevented the "unused variable warning".

Allow materialization strategy when collations on the
inner and outer sides of an IN subquery are the same and the
character set of the inner side is a proper subset of the character
set on the outer side.
This allows conversion from utf8mb3 to utf8mb4
as the former is a subset of the later.
This is only allowed when IN predicate is converted to an IN subquery

Backported part of the patch (d6a00d9b18f) of MDEV-17905.