Changed parsing for unions

[ritwizsinha]

Description

This pull request, makes changes to the select and union grammar in Vitess to match what is implemented in the MYSQL grammar. Here are the changes that have been made

• The parser no longer accepts queries with limit/lock/order by in select with unions without parenthesis for example

select 1 order by 1 union select 2 order by 2

is a parsing error and this type of query is only accepted if it is parenthesized

(select 1 order by 1) union (select 2 order by 2)

• We no longer need to have parenthesis in any other select statement apart from the ones mentioned above when working with unions
• We no longer use ParenSelect struct to put parenthesis around statements but we check if the statement needs parenthesis in the ast_format file, and put parenthesis when needed. All utility functions for ParenSelect have been removed
• The Select struct now accepts an additional field of into clause which is checked and passed when present as specified in the grammar
• The union is now stored recursively with first statement field specifying the left select statement and the right side of union being specified by the UnionSelects field
• Introduced precedence symbols SUBQUERY_AS_EXPR and EMPTY_FROM_CLAUSE to avoid ambiguity in grammar while parsing subqueries and into selects respectively. The same trick has been used and explained in the MYSQL grammar
• Updated the parser and planner tests to reflect the current output of parser.

Related Issue(s)

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[GuptaManan100]

LGTM! 🤟