Implement support for the new .NET LeftJoin operator #35451
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This is pretty straightforward - this removes our internal "fake" LeftJoin method, replacing it with the new one introduced into System.Linq; the existing preprocessing conversion of GroupJoin+SelectMany+DefaultIfEmpty to LeftJoin works exactly as before (and will continue to do so).
One tricky aspect here is the tests - all tests exercising left join are currently written using the current GroupJoin+SelectMany+DefaultIfEmpty syntax. Ideally, left join tests should use the new LeftJoin operator, with some specific, targeted tests verifying that the conversion from the old syntax to LeftJoin still works; but this is quite a lot of work. I've changed several easy/obvious tests to use LeftJoin, but have left most tests as-is for now.
In any case, since GroupJoin+SelectMany+DefaultIfEmpty is already well-covered and converts to LeftJoin, LeftJoin is also well-covered.
Closes #12793