*: fix in-compatible behavior when modify value from Navicat GUI

[XuHuaiyu]

This commit fix 2 things which will affect modify values from Navicat GUI,
and remove comparison of FieldType.Flag when mergeFiledType for Union. :

1. add syntax support for Group_concat( order by)
   Before this commit:

tidb(127.0.0.1)> SELECT student_name, GROUP_CONCAT(DISTINCT test_score ORDER BY test_score DESC SEPARATOR ' ') FROM
 student GROUP BY student_name;
ERROR 1105 (HY000): line 1 column 59 near " BY test_score DESC SEPARATOR ' ') FROM student GROUP BY student_name" (
total length 128)

2. fix the type infer of numeric type columns
   Before this commit:

create table t1(a int, b decimal(15,2));
select * from t1;

MySQL:

Field   1:  `a`
Catalog:    `def`
Database:   `test`
Table:      `t1`
Org_table:  `t1`
Type:       LONG
Collation:  binary (63)
Length:     11
Max_length: 0
Decimals:   0
Flags:      NUM       <== No BinaryFlag

Field   2:  `b`
Catalog:    `def`
Database:   `test`
Table:      `t1`
Org_table:  `t1`
Type:       NEWDECIMAL
Collation:  binary (63)
Length:     17
Max_length: 0
Decimals:   2
Flags:      NUM       <== No BinaryFlag

TiDB:

Field   1:  `a`
Catalog:    `def`
Database:   `test`
Table:      `t1`
Org_table:  `t1`
Type:       LONG
Collation:  binary (63)
Length:     44
Max_length: 0
Decimals:   0
Flags:      BINARY NUM     

Field   2:  `b`
Catalog:    `def`
Database:   `test`
Table:      `t1`
Org_table:  `t1`
Type:       NEWDECIMAL
Collation:  binary (63)
Length:     17
Max_length: 0
Decimals:   2
Flags:      BINARY NUM      

[XuHuaiyu]

PTAL @zz-jason @coocood

[XuHuaiyu]

/run-all-tests

[jackysp]

I think it is strange to comment inside a statement. Could we explain this above line 57?

[XuHuaiyu]

PTAL @jackysp

[zz-jason]

LGTM

[jackysp]

LGTM