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Rollup of 12 pull requests

Successful merges:

 - rust-lang#73140 (Fallback to xml.etree.ElementTree)
 - rust-lang#73670 (Add `format_args_capture` feature)
 - rust-lang#73693 (Use exhaustive match in const_prop.rs)
 - rust-lang#73845 (Use &raw in A|Rc::as_ptr)
 - rust-lang#73861 (Create E0768)
 - rust-lang#73881 (Standardize bibliographic citations in rustc API docs)
 - rust-lang#73925 (Improve comments from rust-lang#72617, as suggested by RalfJung)
 - rust-lang#73949 ([mir-opt] Fix mis-optimization and other issues with the SimplifyArmIdentity pass)
 - rust-lang#73984 (Edit docs for rustc_data_structures::graph::scc)
 - rust-lang#73985 (Fix "getting started" link)
 - rust-lang#73997 (fix typo)
 - rust-lang#73999 (Bump mingw-check CI image from Ubuntu 16.04 to 18.04.)

Failed merges:

 - rust-lang#74000 (add `lazy_normalization_consts` feature gate)

r? @ghost

....to allow use with unsized T. This is a follow-up to rust-lang#73845,
which did the impl work required to be able to relax these bounds.

Allow Weak::as_ptr and friends for unsized T

Relaxes `impl<T> Weak<T>` to `impl<T: ?Sized> Weak<T>` for the methods `rc::Weak::as_ptr`, `into_raw`, and `from_raw`.

Follow-up to rust-lang#73845, which did most of the impl work to make these functions work for `T: ?Sized`.

We still have to adjust the implementation of `Weak::from_raw` here, however, because I missed a use of `ptr.is_null()` previously. This check was necessary when `into`/`from_raw` were first implemented, as `into_raw` returned `ptr::null()` for dangling weak. However, we now just (wrapping) offset dangling weaks' pointers the same as nondangling weak, so the null check is no longer necessary (or even hit). (I can submit just 17a928f as a separate PR if desired.)

As a nice side effect, moves the `fn is_dangling` definition closer to `Weak::new`, which creates the dangling weak.

This technically stabilizes that "something like `align_of_val_raw`" is possible to do. However, I believe the part of the functionality required by these methods here -- specifically, getting the alignment of a pointee from a pointer where it may be dangling iff the pointee is `Sized` -- is uncontroversial enough to stabilize these methods without a way to implement them on stable Rust.

r? @RalfJung, who reviewed rust-lang#73845.

ATTN: This changes (relaxes) the (input) generic bounds on stable fn!

Allow Weak::as_ptr and friends for unsized T

Relaxes `impl<T> Weak<T>` to `impl<T: ?Sized> Weak<T>` for the methods `rc::Weak::as_ptr`, `into_raw`, and `from_raw`.

Follow-up to rust-lang#73845, which did most of the impl work to make these functions work for `T: ?Sized`.

We still have to adjust the implementation of `Weak::from_raw` here, however, because I missed a use of `ptr.is_null()` previously. This check was necessary when `into`/`from_raw` were first implemented, as `into_raw` returned `ptr::null()` for dangling weak. However, we now just (wrapping) offset dangling weaks' pointers the same as nondangling weak, so the null check is no longer necessary (or even hit). (I can submit just 17a928f as a separate PR if desired.)

As a nice side effect, moves the `fn is_dangling` definition closer to `Weak::new`, which creates the dangling weak.

This technically stabilizes that "something like `align_of_val_raw`" is possible to do. However, I believe the part of the functionality required by these methods here -- specifically, getting the alignment of a pointee from a pointer where it may be dangling iff the pointee is `Sized` -- is uncontroversial enough to stabilize these methods without a way to implement them on stable Rust.

r? `@RalfJung,` who reviewed rust-lang#73845.

ATTN: This changes (relaxes) the (input) generic bounds on stable fn!

This was missed in rust-lang#73845 when switching to use the raw operators.
Fixes rust-lang#80365

Use raw version of align_of in rc data_offset

This was missed in rust-lang#73845 when switching to use the raw operators.
Fixes rust-lang#80365