Simplify caseup() and casedn() in charsets

After the MDEV-13118 fix there's no code in the server that
wants caseup/casedn to change the argument in place for simple
charsets.  Let's remove this logic and always return the result in a
new string for all charsets, both simple and complex.

1. Removing the optimization that *some* character sets used in casedn()
  and caseup(), which allowed (and required) to change the case in-place,
  overwriting the string passed as the "src" argument.
  Now all CHARSET_INFO's work in the same way:
  non of them change the source string in-place, all of them now convert
  case from the source string to the destination string, leaving
  the source string untouched.

2. Adding "const" qualifier to the "char *src" parameter
   to caseup() and casedn().

3. Removing duplicate implementations in ctype-mb.c.
  Now both caseup() and casedn() implementations for all CJK character sets
  use internally the same function my_casefold_mb()
  (the former my_casefold_mb_varlen()).

4. Removing the "unused" attribute from parameters of some my_case{up|dn}_xxx()
   implementations, as the affected parameters are now *used* in the code.
   Previously these parameters were used only in DBUG_ASSERT().

1 files changed, 4 insertions, 4 deletions

diff --git a/strings/ctype-ujis.c b/strings/ctype-ujis.c
index e7dbefe6c1d..25f8881d819 100644
--- a/strings/ctype-ujis.c
+++ b/strings/ctype-ujis.c
@@ -67179,12 +67179,12 @@ get_case_info_for_ch(CHARSET_INFO *cs, uint plane, uint page, uint offs)
 */
 static size_t
 my_casefold_ujis(CHARSET_INFO *cs,
-                 char *src, size_t srclen,
+                 const char *src, size_t srclen,
                  char *dst, size_t dstlen __attribute__((unused)),
                  const uchar * const map,
                  size_t is_upper)
 {
-  char *srcend= src + srclen, *dst0= dst;
+  const char *srcend= src + srclen, *dst0= dst;
 
   while (src < srcend)
   {
@@ -67226,7 +67226,7 @@ my_casefold_ujis(CHARSET_INFO *cs,
   LOWER()
 */
 size_t
-my_casedn_ujis(CHARSET_INFO * cs, char *src, size_t srclen,
+my_casedn_ujis(CHARSET_INFO * cs, const char *src, size_t srclen,
                char *dst, size_t dstlen)
 {
   DBUG_ASSERT(dstlen >= srclen * cs->casedn_multiply); 
@@ -67239,7 +67239,7 @@ my_casedn_ujis(CHARSET_INFO * cs, char *src, size_t srclen,
   UPPER()
 */
 size_t
-my_caseup_ujis(CHARSET_INFO * cs, char *src, size_t srclen,
+my_caseup_ujis(CHARSET_INFO * cs, const char *src, size_t srclen,
                char *dst, size_t dstlen)
 {
   DBUG_ASSERT(dstlen >= srclen * cs->caseup_multiply);