Support grouping sets parsing for GenericDialect

[Jefffrey]

Part of apache/datafusion#2469

To allow GenericDialect to parse Grouping sets.

Note that will also now parse rollup & cube as their Expr enum variant now, instead of as Expr::Function

[alamb]

Thank you @Jefffrey

[coveralls]

Pull Request Test Coverage Report for Build 3775315706

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.336%

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Totals
Change from base Build 3720056662:	0.0%
Covered Lines:	12770
Relevant Lines:	14791

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💛 - Coveralls