MDEV-12441 Variables declared after cursors with parameters lose values

Parse context frames (sp_pcontext) can have holes in variable run-time offsets,
the missing offsets reside on the children contexts in such cases.

Example:

  CREATE PROCEDURE p1() AS
    x0 INT:=100;        -- context 0, position 0, run-time 0
    CURSOR cur(
      p0 INT,           -- context 1, position 0, run-time 1
      p1 INT            -- context 1, position 1, run-time 2
    ) IS SELECT p0, p1;
    x1 INT:=101;        -- context 0, position 1, run-time 3
  BEGIN
    ...
  END;

Fixing a few methods to take this into account:
- sp_pcontext::find_variable()
- sp_pcontext::retrieve_field_definitions()
- LEX::sp_variable_declarations_init()
- LEX::sp_variable_declarations_finalize()
- LEX::sp_variable_declarations_rowtype_finalize()
- LEX::sp_variable_declarations_with_ref_finalize()

Adding a convenience method:

  sp_pcontext::get_last_context_variable(uint offset_from_the_end);

to access variables from the end, rather than from the beginning.
This helps to loop through the context variable array (m_vars)
on the fragment that does not have any holes.

Additionally, renaming sp_pcontext::find_context_variable() to
sp_pcontext::get_context_variable(). This method simply returns
the variable by its index. So let's rename to avoid assumptions
that some heavy lookup is going on inside.

mysql-test/suite/compat/oracle/r/sp-code.result

@@ -1348,3 +1348,136 @@ rec1.a
 rec2.a
 11
 DROP PROCEDURE p1;
+#
+# MDEV-12441 Variables declared after cursors with parameters lose values
+#
+CREATE PROCEDURE p1() AS
+x0 INT:=100;
+CURSOR cur(cp1 INT, cp2 INT) IS SELECT cp1+cp2;
+x1 INT:=101;
+BEGIN
+OPEN cur(10,11);
+CLOSE cur;
+SELECT x0, x1;
+END;
+$$
+SHOW PROCEDURE CODE p1;
+Pos	Instruction
+0	set x0@0 100
+1	set x1@3 101
+2	cpush cur@0
+3	set cp1@1 10
+4	set cp2@2 11
+5	copen cur@0
+6	cclose cur@0
+7	stmt 0 "SELECT x0, x1"
+8	cpop 1
+CALL p1();
+x0	x1
+100	101
+DROP PROCEDURE p1;
+CREATE PROCEDURE p1() AS
+x0 INT:=100;
+CURSOR cur0(cp1 INT, cp2 INT) IS SELECT cp1+cp2;
+x1 INT:=101;
+CURSOR cur1(cp1 INT, cp2 INT) IS SELECT cp1+cp2;
+x2 INT:=102;
+CURSOR cur2(cp1 INT, cp2 INT) IS SELECT cp1+cp2;
+x3 INT:=103;
+BEGIN
+OPEN cur0(0,1);
+CLOSE cur0;
+SELECT x0, x1, x2, x3;
+OPEN cur1(10,11);
+CLOSE cur1;
+SELECT x0, x1, x2, x3;
+OPEN cur2(20,21);
+CLOSE cur2;
+SELECT x0, x1, x2, x3;
+END;
+$$
+SHOW PROCEDURE CODE p1;
+Pos	Instruction
+0	set x0@0 100
+1	set x1@3 101
+2	set x2@6 102
+3	set x3@9 103
+4	cpush cur0@0
+5	cpush cur1@1
+6	cpush cur2@2
+7	set cp1@1 0
+8	set cp2@2 1
+9	copen cur0@0
+10	cclose cur0@0
+11	stmt 0 "SELECT x0, x1, x2, x3"
+12	set cp1@4 10
+13	set cp2@5 11
+14	copen cur1@1
+15	cclose cur1@1
+16	stmt 0 "SELECT x0, x1, x2, x3"
+17	set cp1@7 20
+18	set cp2@8 21
+19	copen cur2@2
+20	cclose cur2@2
+21	stmt 0 "SELECT x0, x1, x2, x3"
+22	cpop 3
+CALL p1();
+x0	x1	x2	x3
+100	101	102	103
+x0	x1	x2	x3
+100	101	102	103
+x0	x1	x2	x3
+100	101	102	103
+DROP PROCEDURE p1;
+CREATE TABLE t1 (a INT);
+CREATE PROCEDURE p1() AS
+x0 INT:=100;
+CURSOR cur(cp1 INT, cp2 INT) IS SELECT cp1+cp2;
+x1 t1.a%TYPE:=101;
+BEGIN
+OPEN cur(10,11);
+CLOSE cur;
+SELECT x0, x1;
+END;
+$$
+SHOW PROCEDURE CODE p1;
+Pos	Instruction
+0	set x0@0 100
+1	set x1@3 101
+2	cpush cur@0
+3	set cp1@1 10
+4	set cp2@2 11
+5	copen cur@0
+6	cclose cur@0
+7	stmt 0 "SELECT x0, x1"
+8	cpop 1
+CALL p1();
+x0	x1
+100	101
+DROP PROCEDURE p1;
+DROP TABLE t1;
+CREATE PROCEDURE p1() AS
+x0 INT:=100;
+CURSOR cur(cp1 INT, cp2 INT) IS SELECT cp1+cp2;
+x1 ROW(a INT,b INT):=ROW(101,102);
+BEGIN
+OPEN cur(10,11);
+CLOSE cur;
+SELECT x0, x1.a, x1.b;
+END;
+$$
+SHOW PROCEDURE CODE p1;
+Pos	Instruction
+0	set x0@0 100
+1	set x1@3 (101,102)
+2	cpush cur@0
+3	set cp1@1 10
+4	set cp2@2 11
+5	copen cur@0
+6	cclose cur@0
+7	stmt 0 "SELECT x0, x1.a, x1.b"
+8	cpop 1
+CALL p1();
+x0	x1.a	x1.b
+100	101	102
+DROP PROCEDURE p1;