move rustc_type_ir Term things to term_kind.rs

[khyperia]

• TyKind, AliasTy, and AliasTyKind live in ty_kind.rs
• ConstKind, UnevaluatedConst, and UnevaluatedConstKind live in const_kind.rs
• TermKind, AliasTerm, and AliasTermKind live in... generic_arg.rs and predicate.rs ???

create a new file, term_kind.rs, that holds TermKind, AliasTerm, and AliasTermKind

(note that Ty, Const, and Term live in rustc_middle, not rustc_type_ir)

[rustbot]

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[Jamesbarford]

This looks like there is quite a lot of overlap with; #156538 ?

[khyperia]

This looks like there is quite a lot of overlap with; #156538 ?

True - feel free to integrate this change with your PR (i.e. moving TermKind/AliasTermKind to term_kind.rs). Otherwise I can wait until your PR lands and rebase this one on that, totally fine either way!

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.