Improve C++ codegen by using last precedence

[namannimmo10]

No description provided.

[certik]

You have to update test results.

[certik]

@namannimmo10, if you want, you can try to finish this one. It shouldn't be difficult.

[namannimmo10]

I'll try to complete this one soon.

[namannimmo10]

Ready? Thanks for your comments!

[certik]

I haven't gone over the other expr visitors, but we need to carefully go over every expr visitor and ensure we assign to last_expr_precedence, or add a comment.

[certik]

Last minor update I found. After that, do you mind rebasing this PR to a more logical set of commits? It looks like we don't have any comments, we always assign the last precedence, so you can probably squash the last few commits together and rewrite the commit log.

[certik]

I think this looks great, thanks! After this is merged, I'll port it to LFortran where we have a lot more C++ tests and if there are any bugs, I'll report back.