Do not attempt to evaluate obligations on PointeeSized goals

[KiChjang]

Fixes #151957.

In this PR, we simply special-case the fulfillment goal if the goal is PointeeSized, since the trait itself isn't lowered to the trait solver, and all types implement it.

[rustbot]

r? @fmease

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[KiChjang]

r? @rperier since you have left comments in an earlier PR

[rustbot]

Failed to set assignee to rperier: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[KiChjang]

Looks like it's not working, so r? @lcnr instead since you've reviewed the PR made by @rperier in #144674.

[theemathas]

Note: #151962 already exists

[rperier]

Yeah, I am not a reviewer. I have just wrote the original feature (reporting similarly named traits) :)

Could you sync with the author of #151962, or co-author the fix with him perhaps, as there was already a PR opened for this issue.

[lcnr]

I unfortunately looked at #151962 first and reviewed that PR already. Your fix for this issue does actually seem more correct, but I would still go with the first one.

Thank you for looking into this and opening this PR!