- MDEV-6695 Bad column name for UCS2 string literals

The Item_string constructors called set_name() on the source string,
  which was wrong because in case of UCS2/UTF16/UTF32 the source value
  might be a not well formed string (e.g. have incomplete leftmost character).
  Now set_name() is called on str_value after its copied 
  (with optionally left zero padding) from the source string.
- MDEV-6694 Illegal mix of collation with a PS parameter
  Item_param::convert_str_value() did not set repertoire.
  Introducing a new structure MY_STRING_METADATA to collect
  character length and repertoire of a string in a single loop,
  to avoid two separate loops. Adding a new class Item_basic_value::Metadata
  as a convenience wrapper around MY_STRING_METADATA, to reuse the
  code between Item_string and Item_param.

sql/sql_string.cc

@@ -580,7 +580,7 @@ bool String::append_with_prefill(const char *s,uint32 arg_length,
   return FALSE;
 }
 
-uint32 String::numchars()
+uint32 String::numchars() const
 {
   return str_charset->cset->numchars(str_charset, Ptr, Ptr+str_length);
 }