planner: GetParamSQLFromAST is not thread-safe

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Use the test below to reproduce:

	n := 100
	sqls := make([]string, 0, n)
	for i := 0; i < n; i++ {
		sqls = append(sqls, fmt.Sprintf(`insert into t values (%d, %d, %d)`, i*3+0, i*3+1, i*3+2))
	}
	stmts := make([]ast.StmtNode, 0, n)
	for _, sql := range sqls {
		stmt, err := parser.New().ParseOneStmt(sql, "", "")
		require.Nil(t, err)
		stmts = append(stmts, stmt)
	}

	var wg sync.WaitGroup
	for i := 0; i < n; i++ {
		wg.Add(1)
		go func(id int) {
			for i := 0; i < 1000; i++ {
				_, vals, err := GetParamSQLFromAST(context.Background(), MockContext(), stmts[id])
				require.Nil(t, err)
				require.Equal(t, len(vals), 3)
				require.Equal(t, vals[0].GetValue(), int64(id*3+0))
				require.Equal(t, vals[1].GetValue(), int64(id*3+1))
				require.Equal(t, vals[2].GetValue(), int64(id*3+2))
				time.Sleep(time.Millisecond + time.Duration(rand.Intn(int(time.Millisecond))))
			}
			wg.Done()
		}(i)
	}
	wg.Wait()

2. What did you expect to see? (Required)

No error

3. What did you see instead (Required)

Can't pass the test, some require.Equal might return errors.

4. What is your TiDB version? (Required)

[qw4990]

Fixed by #43242