Add a new linting pass for obligations

[ghost]

(Note: this is currently just a rough cut of the idea being sent in for initial discussion: zulip stream. Nothing in here is properly feature gated.)

This adds a new linting pass for when obligations have been used.

This has been spun out as a prerequisite of #95025, where such a capability was needed in order to lint when #[deprecated_safe] unsafe fn() -> closure coercions were used. (Here's an alternate reality branch that implements #95025 using the system being proposed here)

@lcnr expressed desire for a similar ability on my review, "mostly to deprecate the Eq and PartialEq impls for function pointers". This example lint has been used in this PR rather than pulling in all the #[deprecated_safe] changes just to test implementing this new system.

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[rust-highfive]

r? @davidtwco

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[ghost]

@jackh726, i think this should be assigned to you

[davidtwco]

r? @jackh726

[bors]

☔ The latest upstream changes (presumably #96892) made this pull request unmergeable. Please resolve the merge conflicts.

[jhpratt]

As the PR's author has deleted their account, I have backed up the current state of the PR on my fork. The code is available here. This should make it easier for someone to use the already-written code if they want to take the PR over.