Add more left join support

[gliljas]

Fixes #2672

LINQ LeftJoin only works if the outer reference is projected in the query result, but it's perfectly feasible to only use it in e.g a where clause.

The new visitor could be merged a bit with the old one, in order to allocate a bit less, but I wanted to keep the old one as is for now, since it's used elsewhere. Perhaps the aggregated group joins should be investigated too.

I added two other failing test cases as well, marked with KnownBug. One which doesn't use the outer reference at all. Not a very likely query, since the join becomes irrelevant, but in query generation scenarios it could happen. I guess a visitor which purges the query model from unused left joins could do the trick.

The other test case runs a nested join. I think that could be rewritten to chain the nested joins, instead of nesting them.

Unfortunately, I wasn't able to run the async generator. "Failure scanning NAnt.NUnit2Tasks.dll for extensions"

[fredericDelaporte]

Unfortunately, I wasn't able to run the async generator. "Failure scanning NAnt.NUnit2Tasks.dll for extensions"

On master branch, this happens if you have kept somewhere a global.json file downgrading the .Net Core SDK.
Since this file is required when generating on 5.3.x branches, I just rename it when I am generating on master branch.

[gliljas]

Aha, thanks!

[maca88]

The new visitor could be merged a bit with the old one

I updated the new visitor to use an instance of the old one to reduce the allocation.

I added two other failing test cases as well, marked with KnownBug. One which doesn't use the outer reference at all

I added a fix for the one that doesn't use the outer reference anywhere as it wasn't that complicated.

[gliljas]

Thanks, @maca88 !

[burhanugur]

Hi @fredericDelaporte @bahusoid ,

I have the same problem with #2672 . This pull request says it has resolved the issue. When do you plan to merge this request?

[fredericDelaporte]

For some unknown reason, GitHub seems to have difficulties getting the TeamCity builds statuses. Checking manually on https://teamcity.jetbrains.com/, they have all succeeded for this PR. So, I consider forcing the merge for this PR.