MDEV-31496: Make optimizer handle UCASE(varchar_col)=...

(Review input addressed)
(Added handling of UPDATE/DELETE and partitioning w/o index)

If the properties of the used collation allow, do the following
equivalent rewrites:

1. UPPER(key_col)=expr  ->  key_col=expr
   expr=UPPER(key_col)  ->  expr=key_col
   (also rewrite both sides of the equality at the same time)

2. UPPER(key_col) IN (constant-list)  -> key_col IN (constant-list)

- Mark utf8mb{3,4}_general_ci as collations that allow this.
- Add optimizer_switch='sargable_casefold=ON' to control this.
  (ON by default in this patch)
- Cover the rewrite in Optimizer Trace, rewrite name is
  "sargable_casefold_removal".

mysql-test/include/sargable_casefold.inc

@@ -0,0 +1,44 @@
+# Check sargable_casefold rewrite for $collation
+
+eval create table t1 (
+  col1 varchar(32), 
+  col2 varchar(32), 
+  col3 char(32), 
+  col4  text, 
+  key(col1),
+  key(col2),
+  key(col3),
+  key(col4(32))
+) collate $collation;
+
+insert into t1
+select 
+  concat('A-', seq),
+  concat('A-', seq),
+  concat('A-', seq),
+  concat('A-', seq)
+from seq_1_to_100;
+
+analyze table t1 persistent for all;
+
+--echo # Basic examples. All should use ref(col1):
+explain
+select * from t1 where upper(col1)='A-3';
+select * from t1 where upper(col1)='A-3';
+
+explain
+select * from t1 where ucase(col1)='a-3';
+select * from t1 where ucase(col1)='a-3';
+
+explain select * from t1 where 'abc'=upper(col1);
+explain select * from t1 where 'xyz'=ucase(col1);
+
+create view v1 as select * from t1;
+explain select * from v1 where 'abc'=upper(col1);
+drop view v1;
+
+explain select * from t1 where upper(col3)='a-3';
+explain select * from t1 where upper(col4)='a-3';
+
+# DROP TABLE t1 is missing intentionally here.
+