Rename some body_id to body_def_id

[camsteffen]

Since body IDs and item IDs are distinct things, try to use names that do not conflate them. body_def_id is a little easier to read as "body-owning definition ID" whereas body_id is hard not to just read as "body ID".

Related: #158611 - I noticed this afterwards. I suspect that FnCtxt ought to keep body_def_id but maybe add body_id: BodyId in addition.

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[oli-obk]

hmm... various of those are not items. The DefId of a const block has a body, but is not an item. I don't know if this really is an improvement even in the few cases where it may be unambiguous. Is there some other discussions that led you to doing this PR?

[camsteffen]

hmm... various of those are not items.

Okay, that's my mistake, I thought they were. How about body_owner_id or def_id or body_def_id? body_id just seems like a misleading misnomer to me. No other discussions to my knowledge, just my own idea.

[camsteffen]

I noticed that body_def_id is used sometimes throughout the codebase, so I changed to using that name instead of body_id more consistently. Admittedly this is now a pretty subtle change that may not be worth the churn. But I do think body_def_id is a little easier to understand as "body-owning definition ID".

[rust-bors]

☔ The latest upstream changes (presumably #158795) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

[oli-obk]

r=me after a rebase