ddl: truncate partition not handled correctly for global index

[L-maple]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

A unit test:

func TestTruncatePartitionWithGlobalIndex(t *testing.T) {
	defer config.RestoreFunc()()
	config.UpdateGlobal(func(conf *config.Config) {
		conf.EnableGlobalIndex = true
	})
	store := testkit.CreateMockStore(t)
	tk := testkit.NewTestKit(t, store)
	tk.MustExec("use test")
	tk.MustExec("drop table if exists test_global")
	tk.MustExec(`create table test_global ( a int, b int, c int)
	partition by range( a ) (
		partition p1 values less than (10),
		partition p2 values less than (20)
	);`)
	tt := external.GetTableByName(t, tk, "test", "test_global")
	pid := tt.Meta().Partition.Definitions[1].ID

	tk.MustExec("Alter Table test_global Add Unique Index idx_b (b);")
	tk.MustExec("Alter Table test_global Add Unique Index idx_c (c);")
	tk.MustExec(`INSERT INTO test_global VALUES (1, 1, 1), (2, 2, 2), (11, 3, 3), (12, 4, 4)`)

	tk.MustExec("alter table test_global truncate partition p2;")
	result := tk.MustQuery("select * from test_global;")
	result.Sort().Check(testkit.Rows(`1 1 1`, `2 2 2`))

	tt = external.GetTableByName(t, tk, "test", "test_global")
	idxInfo := tt.Meta().FindIndexByName("idx_b")
	require.NotNil(t, idxInfo)
	cnt := checkGlobalIndexCleanUpDone(t, tk.Session(), tt.Meta(), idxInfo, pid)
	require.Equal(t, 2, cnt)

	idxInfo = tt.Meta().FindIndexByName("idx_c")
	require.NotNil(t, idxInfo)
	cnt = checkGlobalIndexCleanUpDone(t, tk.Session(), tt.Meta(), idxInfo, pid)
	require.Equal(t, 2, cnt)
}

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

pass

3. What did you see instead (Required)

failed, because global index rows was not cleaned.

4. What is your TiDB version? (Required)