babelfish-for-postgresql / babelfish_extensions / 11071542710

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Fix Antlr rewrite for WITHIN GROUP() for STRING_AGG() (#2983)

### 1. Issues
- Rewriting logic in antlr of WITHIN GROUP() for STRING_AGG() was incorrect as it involved shifting of order_by_clause. This will cause a problem when there is rewrite in order_by_clause itself as now the index of expressions inside order_by_clause will change and this will result in incorrect rewrite.

### 2. Changes made to fix the issues
- Corrected Parser rewiting logic to avoid shifting of order_by_clause and only omit the unwanted phrases in between.
```
# Input
1> CREATE TABLE string_agg_t (id int, a varchar(10), b varchar(10), g int, sbid int)
2> INSERT INTO string_agg_t values (3,'c','x',1,4), (2,'b','y',2,6)
3> go
1> SELECT STRING_AGG(concat(a,b),'-') WITHIN GROUP (ORDER BY trim(a) ASC) FROM string_agg_t GROUP BY g ORDER BY g
2> go
```
```
# previous output (wrong output)               
Msg 33557097, Level 16, State 1, Server BABELFISH, Line 1
syntax error at or near ")" 
```
```
# current output (correct output)

string_agg                                                                                                                                                                                                                                                      
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
cx                                                                                                                                                                                                                                                              
by                                                                                                                                                                             ... (continued)

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