Rollup merge of #97739 - a2aaron:let_underscore, r=estebank

Uplift the `let_underscore` lints from clippy into rustc.

This PR resolves #97241.

This PR adds three lints from clippy--`let_underscore_drop`, `let_underscore_lock`, and `let_underscore_must_use`, which are meant to capture likely-incorrect uses of `let _ = ...` bindings (in particular, doing this on a type with a non-trivial `Drop` causes the `Drop` to occur immediately, instead of at the end of the scope. For a type like `MutexGuard`, this effectively releases the lock immediately, which is almost certainly the wrong behavior)

In porting the lints from clippy I had to copy over a bunch of utility functions from `clippy_util` that these lints also relied upon. Is that the right approach?

Note that I've set the `must_use` and `drop` lints to Allow by default and set `lock` to Deny by default (this matches the same settings that clippy has). In talking with `@estebank` he informed me to do a Crater run (I am not sure what type of Crater run to request here--I think it's just "check only"?)

On the linked issue, there's some discussion about using `must_use` and `Drop` together as a heuristic for when to warn--I did not implement this yet.

r? `@estebank`

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