MDEV-25829 Change default Unicode collation to uca1400_ai_ci

Step#2 - Adding a new collation derivation level for CAST and CONVERT.

Now character string cast functions:
  - CAST(string_expr AS CHAR)
  - CONVERT(expr USING charset_name)

have a new collation derivation level between:

  - string literals
  - utf8 metadata functions, e.g. user() and database()

Before the change these cast functions had collation derivation equal
to table columns, which caused more illegal mix of collation conflicts.

Note, binary string cast functions:
  - BINARY(expr)
  - CAST(string_expr AS BINARY)
  - CONVERT(expr USING binary)
did not change their collation derivation, to preserve the behaviour of
queries like these:
SELECT database()=BINARY'test';
SELECT user()=CAST('root' AS BINARY);
SELECT current_role()=CONVERT('role' USING binary);

Derivation levels after the change look as follows:

  DERIVATION_IGNORABLE= 7, // Explicit NULL

  DERIVATION_NUMERIC= 6,   // Numbers in string context,
                           // Numeric user variables
                           // CAST(numeric_expr AS CHAR)

  DERIVATION_COERCIBLE= 5, // Literals, string user variables

  DERIVATION_CAST= 4,      // CAST(string_expr AS CHAR),
                           // CONVERT(string_expr USING cs)

  DERIVATION_SYSCONST= 3,  // utf8 metadata functions, e.g. user(), database()
  DERIVATION_IMPLICIT= 2,  // Table columns, SP variables, BINARY(expr)
  DERIVATION_NONE= 1,      // A mix (e.g. CONCAT) of two differrent collations
  DERIVATION_EXPLICIT= 0   // An explicit COLLATE clause

sql/sql_type.h

@@ -3019,13 +3019,46 @@ static inline my_repertoire_t &operator|=(my_repertoire_t &a,
 
 enum Derivation
 {
-  DERIVATION_IGNORABLE= 6,
-  DERIVATION_NUMERIC= 5,
-  DERIVATION_COERCIBLE= 4,
+  DERIVATION_IGNORABLE= 7, // Explicit NULL
+
+  /*
+    Explicit or implicit conversion from numeric/temporal data to string:
+    - Numbers/temporals in string context
+    - Numeric user variables
+    - CAST(numeric_or_temporal_expr AS CHAR)
+  */
+  DERIVATION_NUMERIC= 6,
+
+  /*
+    - String literals
+    - String user variables
+  */
+  DERIVATION_COERCIBLE= 5,
+
+  /*
+    String cast and conversion functions:
+    - BINARY(expr)
+    - CAST(string_expr AS CHAR)
+    - CONVERT(expr USING cs)
+  */
+  DERIVATION_CAST= 4,
+
+  /*
+    utf8 metadata functions:
+    - DATABASE()
+    - CURRENT_ROLE()
+    - USER()
+  */
   DERIVATION_SYSCONST= 3,
+
+  /*
+    - Table columns
+    - SP variables
+    - BINARY(expr) and CAST(expr AS BINARY)
+  */
   DERIVATION_IMPLICIT= 2,
-  DERIVATION_NONE= 1,
-  DERIVATION_EXPLICIT= 0
+  DERIVATION_NONE= 1,      // A mix (e.g. CONCAT) of two differrent collations
+  DERIVATION_EXPLICIT= 0   // An explicit COLLATE clause
 };
 
 
@@ -3079,6 +3112,12 @@ public:
     derivation(derivation_arg),
     repertoire(repertoire_arg)
   { }
+  static DTCollation string_typecast(CHARSET_INFO *collation_arg)
+  {
+    return DTCollation(collation_arg,
+                       collation_arg == &my_charset_bin ?
+                       DERIVATION_IMPLICIT : DERIVATION_CAST);
+  }
   void set(const DTCollation &dt)
   {
     *this= dt;
@@ -3114,6 +3153,7 @@ public:
       case DERIVATION_NUMERIC:   return "NUMERIC";
       case DERIVATION_IGNORABLE: return "IGNORABLE";
       case DERIVATION_COERCIBLE: return "COERCIBLE";
+      case DERIVATION_CAST:      return "CAST";
       case DERIVATION_IMPLICIT:  return "IMPLICIT";
       case DERIVATION_SYSCONST:  return "SYSCONST";
       case DERIVATION_EXPLICIT:  return "EXPLICIT";