This commit adds a appear-on-over link to all section headers to generated
documentation. Each header also receives an id now, even those generated through
markdown. The purpose of this is to provide easy to link to sections.

This modifies the default header markdown generation because the default id
added looks like "toc_NN" which is difficult to reconcile among all sections (by
default each section gets a "toc_0" id), and it's also not very descriptive of
where you're going.

This chooses to adopt the github-style anchors by taking the contents of the
title and hyphen-separating them (after lower casing).

Closes #12681

Bump deps

Let `qualify_min_const_fn` deal with drop terminators

Fixes rust-lang#12677

The `method_accepts_droppable` check that was there seemed overly conservative.
> Returns true if any of the method parameters is a type that implements `Drop`.
> The method can't be made const then, because `drop` can't be const-evaluated.

Accepting parameters that implement `Drop` should still be fine as long as the parameter isn't actually dropped, as is the case in the linked issue where the droppable is moved into the return place. This more accurate analysis ("is there a `drop` terminator") is already done by `qualify_min_const_fn` [here](https://github.com/rust-lang/rust-clippy/blob/f5e250180c342bb52808c9a934c962a8fe40afc7/clippy_utils/src/qualify_min_const_fn.rs#L298), so I don't think this additional check is really necessary?

Fixing the other, second case in the linked issue was only slightly more involved, since `Vec::new()` is a function call that has the ability to panic, so there must be a `drop()` terminator for cleanup, however we should be able to freely ignore that. [Const checking ignores cleanup blocks](https://github.com/rust-lang/rust/blob/master/compiler/rustc_const_eval/src/transform/check_consts/check.rs#L382-L388), so we should, too?

r? `@Jarcho`

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changelog: [`missing_const_for_fn`]: continue linting on fns with parameters implementing `Drop` if they're not actually dropped