MyXQL: Add back subquery parens inside of function calls

[greg-rychlewski]

Closes https://github.com/elixir-ecto/ecto_sql/issues/642

This was removed when subquery support was added to order by/group by. I believe it was because stuff like exists((..subquery...)) looked weird and not necessarily because it breaks the query. All my local tests passed and all our integration tests pass, including the ones added when this change was made.

I opted for adding the parens back because the other option is to special case certain functions. Which I think can get hairy.

[josevalim]

Should we add a test with the coalesce example in #642?