Avoid emitting `assigning_clones` when cloned data borrows from the place to clone into

[y21]

Fixes #12444 Fixes #12460 Fixes #12749 Fixes #12757

I think the documentation for the function should describe what- and how this is fixing the issues well. It avoids emitting a warning when the data being cloned borrows from the place to clone into, which is information that we can get from PossibleBorrowerMap. Unfortunately, it is a tiny bit tedious to match on the MIR like that and I'm not sure if this is possibly relying a bit too much on the exact MIR lowering for assignments.

Things left to do:

• [x] Handle place projections (or verify that they work as expected)
• [x] Handle non-Drop types

changelog: [assigning_clones]: avoid warning when the suggestion would lead to a borrow-check error

[rustbot]

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[bors]

:umbrella: The latest upstream changes (presumably #12783) made this pull request unmergeable. Please resolve the merge conflicts.

[kornelski]

It would be great to merge this. Even if it's not perfect, it's much better than what has shipped in stable and causing numerous annoyances.

[Alexendoo]

@bors r+

It seems reasonable to me, it is relying on the MIR layout a bit but it seems like it would be fairly straightforward to adapt if that ever changes

[bors]

:pushpin: Commit 60508f546a07fa51169736766b7e16676724c496 has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 60508f546a07fa51169736766b7e16676724c496 with merge 0dc265ff82fe861f673f20003e4370df52f50ff3...

[bors]

:sunny: Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test Approved by: Alexendoo Pushing 0dc265ff82fe861f673f20003e4370df52f50ff3 to master...