MDEV-34189 Unexpected error on WHERE inet6col

normalize_cond() translated `WHERE col` into `WHERE col<>0`

But the opetator "not equal to 0" does not necessarily exists
for all data types.

For example, the query:

  SELECT * FROM t1 WHERE inet6col;

was translated to:

  SELECT * FROM t1 WHERE inet6col<>0;

which further failed with this error:

  ERROR : Illegal parameter data types inet6 and bigint for operation '<>'

This patch changes the translation from `col<>0` to `col IS TRUE`.
So now
  SELECT * FROM t1 WHERE inet6col;
gets translated to:
  SELECT * FROM t1 WHERE inet6col IS TRUE;

Details:
1. Implementing methods:
   - Field_longstr::val_bool()
   - Field_string::val_bool()
   - Item::val_int_from_val_str()
   If the input contains bad data,
   these methods raise a better error message:
     Truncated incorrect BOOLEAN value
   Before the change, the error was:
     Truncated incorrect DOUBLE value

2. Fixing normalize_cond() to generate Item_func_istrue/Item_func_isfalse
   instances instead of Item_func_ne/Item_func_eq

3. Making Item_func_truth sargable, so it uses the range optimizer.
   Implementing the following methods:
   - get_mm_tree(), get_mm_leaf(), add_key_fields() in Item_func_truth.
   - get_func_mm_tree(), for all Item_func_truth descendants.

4. Implementing the method negated_item() for all Item_func_truth
   descendants, so the negated item has a chance to be sargable:
   For example,
     WHERE NOT col IS NOT FALSE    -- this notation is not sargable
   is now translated to:
     WHERE col IS FALSE            -- this notation is sargable

sql/field.cc

@@ -1132,6 +1132,20 @@ Field_longstr::pack_sort_string(uchar *to, const SORT_FIELD_ATTR *sort_field)
 }
 
 
+bool Field_longstr::val_bool()
+{
+  DBUG_ASSERT(marked_for_read());
+  THD *thd= get_thd();
+  StringBuffer<STRING_BUFFER_USUAL_SIZE> str;
+  val_str(&str, &str);
+  double res= Converter_strntod_with_warn(thd, Warn_filter(thd),
+                                          "BOOLEAN",
+                                          charset(),
+                                          str.ptr(), str.length()).result();
+  return res != 0.0;
+}
+
+
 /**
   @brief
   Determine the relative position of the field value in a numeric interval
@@ -7682,11 +7696,26 @@ double Field_string::val_real(void)
   const LEX_CSTRING str= to_lex_cstring();
   return Converter_strntod_with_warn(thd,
                                      Warn_filter_string(thd, this),
+                                     "DOUBLE",
                                      Field_string::charset(),
                                      str.str, str.length).result();
 }
 
 
+bool Field_string::val_bool()
+{
+  DBUG_ASSERT(marked_for_read());
+  THD *thd= get_thd();
+  const LEX_CSTRING str= to_lex_cstring();
+  double res= Converter_strntod_with_warn(thd,
+                                          Warn_filter_string(thd, this),
+                                          "BOOLEAN",
+                                          Field_string::charset(),
+                                          str.str, str.length).result();
+  return res != 0.0;
+}
+
+
 longlong Field_string::val_int(void)
 {
   DBUG_ASSERT(marked_for_read());
@@ -8090,7 +8119,7 @@ double Field_varstring::val_real(void)
 {
   DBUG_ASSERT(marked_for_read());
   THD *thd= get_thd();
-  return Converter_strntod_with_warn(thd, Warn_filter(thd),
+  return Converter_strntod_with_warn(thd, Warn_filter(thd), "DOUBLE",
                                      Field_varstring::charset(),
                                      (const char *) get_data(),
                                      get_length()).result();
@@ -8702,7 +8731,8 @@ double Field_varstring_compressed::val_real(void)
   THD *thd= get_thd();
   String buf;
   val_str(&buf, &buf);
-  return Converter_strntod_with_warn(thd, Warn_filter(thd), field_charset(),
+  return Converter_strntod_with_warn(thd, Warn_filter(thd), "DOUBLE",
+                                     field_charset(),
                                      buf.ptr(), buf.length()).result();
 }
 
@@ -8926,7 +8956,7 @@ double Field_blob::val_real(void)
   if (!blob)
     return 0.0;
   THD *thd= get_thd();
-  return  Converter_strntod_with_warn(thd, Warn_filter(thd),
+  return  Converter_strntod_with_warn(thd, Warn_filter(thd), "DOUBLE",
                                       Field_blob::charset(),
                                       blob, get_length(ptr)).result();
 }
@@ -9394,7 +9424,8 @@ double Field_blob_compressed::val_real(void)
   THD *thd= get_thd();
   String buf;
   val_str(&buf, &buf);
-  return Converter_strntod_with_warn(thd, Warn_filter(thd), field_charset(),
+  return Converter_strntod_with_warn(thd, Warn_filter(thd), "DOUBLE",
+                                     field_charset(),
                                      buf.ptr(), buf.length()).result();
 }