Update unstable docs for `asm!` macro

This adds documentation that SPIR-V is supported, expands on the restrictions for labels, and has some minor cleanups or clarifications.

r? `@joshtriplett`

Fix: don't document private macros by default

As part of #88019, I made it so private macros are documented in `--document-private-items` mode. Unfortunately, it appears that I also accidentally made them be documented when *not* in `--document-private-items` mode. This PR fixes that and adds a regression test.

r? `@jyn514` (I hope you don't mind that I keep sending PRs your way)

Fixes #88453.

rustc_target: `TyAndLayout::field` should never error.

This refactor (making `TyAndLayout::field` return `TyAndLayout` without any `Result` around it) is based on a simple observation, regarding `TyAndLayout::field`:

If `cx.layout_of(ty)` succeeds (for some `cx` and `ty`), then `.field(cx, i)` on the resulting `TyAndLayout` should *always* succeed in computing `cx.layout_of(field_ty)` (where `field_ty` is the type of the `i`th field of `ty`).

The reason for this is that no matter which field is chosen, `cx.layout_of(field_ty)` *will have already been computed*, as part of computing `cx.layout_of(ty)`, as we cannot determine the layout of *any* type without considering the layouts of *all* of its fields.

And so it should be fine to turn any errors into ICEs, since they likely indicate a `cx` mismatch, or some other edge case that is due to a compiler bug (as opposed to ever being an user-facing error).

<hr/>

Each commit should probably be reviewed separately, though note that there's some `where` clauses (in `rustc_target::abi::call::*`) that change in most commits.

cc `@nagisa` `@oli-obk`

Handle match statements with non exhaustive variants in closures

This PR ensures that the behavior for match statements with non exhaustive variants is the same inside and outside closures.

If we have a non-exhaustive SingleVariant which is defined in a different crate, then we should handle the case the same way we would handle a MultiVariant: borrow the match discriminant.

Closes https://github.com/rust-lang/project-rfc-2229/issues/59
r? `@nikomatsakis`

Cow'ify some pprust methods

Reduce number of potential needless de/allocations by using `Cow<'static, str>` instead of explicit `String` type.

Rollup of 13 pull requests

Successful merges:

 - #80543 (Notify when an `I-prioritize` issue is closed or reopened)
 - #83251 (Suggestion for call on immutable binding of mutable type)
 - #85534 (add rustc-demangle assertion on mangled symbol)
 - #88173 (Refactor Markdown length-limited summary implementation)
 - #88349 (Add const and static TAIT tests)
 - #88357 (add unsized coercion test)
 - #88381 (Handle stack_t.ss_sp type change for DragonFlyBSD)
 - #88387 (Remove vestigial rustfix tests.)
 - #88396 (Bump vulnerable crates)
 - #88407 (Fix formatting in release notes from 52a98834)
 - #88411 (Remove `Session.if_let_suggestions`)
 - #88417 (RELEASES.md: fix broken link)
 - #88419 (Fix code blocks color in Ayu theme)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Fix code blocks color in Ayu theme

Fixes #88415.

cc `@camelid`
r? `@notriddle`

RELEASES.md: fix broken link

Remove `Session.if_let_suggestions`

We can instead if either the LHS or RHS types contain
`TyKind::Error`. In addition to covering the case where
we would have previously updated `if_let_suggestions`, this might
also prevent redundant errors in other cases as well.

Fix formatting in release notes from 52a98834

I neglected to add a line that allowed the `[cargo/9663]` short-hand to resolve to an actual link in the rendered markdown on github.

Bump vulnerable crates

crossbeam-deque v0.7.3 -> v0.7.4:
    https://rustsec.org/advisories/RUSTSEC-2021-0093
    https://github.com/crossbeam-rs/crossbeam/pull/728/files

openssl-src v111.15.0+1.1.1k -> v111.16.0+1.1.1l:
    https://rustsec.org/advisories/RUSTSEC-2021-0097
    https://rustsec.org/advisories/RUSTSEC-2021-0098
    https://www.openssl.org/news/vulnerabilities-1.1.1.html

tar v0.4.35 -> v0.4.37:
    https://rustsec.org/advisories/RUSTSEC-2021-0080
    updated to 0.4.37 as there breaking change in 0.4.36: https://github.com/alexcrichton/tar-rs/pull/260

Remove vestigial rustfix tests.

The directory `src/test/rustfix` is not actually tested. It looks like a mistake was made when rustfix tests were originally introduced in #50084.  In commit 6f2d0230 they were moved to `src/test/ui`, but the tests in the original directory weren't deleted.

Handle stack_t.ss_sp type change for DragonFlyBSD

stack_t.ss_sp is now c_void on DragonFlyBSD, like the rest of the BSDs.

Changed in https://github.com/rust-lang/libc/commit/02922ef7504906589d02c2e4d97d1172fa247cc3.

add unsized coercion test

we had no tests in our test suite for this case

Add const and static TAIT tests

r? ``@oli-obk``

Related to #86727

Refactor Markdown length-limited summary implementation

This PR is a new approach to #79749.

This PR refactors the implementation of `markdown_summary_with_limit()`,
separating the logic of determining when the limit has been reached from
the actual rendering process.

The main advantage of the new approach is that it guarantees that all
HTML tags are closed, whereas the previous implementation could generate
tags that were never closed. It also ensures that no empty tags are
generated (e.g., `<em></em>`).

The new implementation consists of a general-purpose struct
`HtmlWithLimit` that manages the length-limiting logic and a function
`markdown_summary_with_limit()` that renders Markdown to HTML using the
struct.

r? `@GuillaumeGomez`

add rustc-demangle assertion on mangled symbol

cc https://github.com/rust-lang/rust/issues/60705#issuecomment-844880365
r? `@michaelwoerister`

Suggestion for call on immutable binding of mutable type

When calling a method requiring a mutable self borrow on an inmutable
to a mutable borrow of the type, suggest making the binding mutable.

Fix #83241.

Notify when an `I-prioritize` issue is closed or reopened

Companion PR to rust-lang/triagebot#1078, blocked on that PR.
r? ``@spastorino`` cc ``@rust-lang/wg-prioritization``

ast_lowering: Introduce `lower_span` for catching all spans entering HIR

This PR cherry-picks the `fn lower_span` change from https://github.com/rust-lang/rust/pull/84373.
I also introduced `fn lower_ident` for lowering spans in identifiers, and audited places where HIR structures with spans or identifiers are constructed and added a few missing `lower_span`s/`lower_ident`s.

Having a hook for spans entering HIR can be useful for things other than https://github.com/rust-lang/rust/pull/84373, e.g. https://github.com/rust-lang/rust/issues/35148.
I also want to check whether this change causes perf regressions due to some accidental inlining issues.

r? `@cjgillot`

Treat types in unnormalized function signatures as well-formed

Fixes #87748

r? `@nikomatsakis`

When calling a method requiring a mutable self borrow on an inmutable
to a mutable borrow of the type, suggest making the binding mutable.

Fix #83241.

Update the stdarch submodule

This notably brings in a number of codegen updates to ensure that wasm
simd intrinsics generate the expected instruction with LLVM 13

Make `-Z gcc-ld=lld` work for Apple targets

`-Z gcc-ld=lld` was introduced in #85961. It does not work on Macos because lld needs be either named `ld64` or passed `-flavor darwin` as the first two arguments in order to select the Mach-O flavor. Rust invokes cc (=clang) on Macos for linking which calls `ld` as linker binary and not `ld64`, so just creating an `ld64` binary and modifying the search path with `-B` does not work.

In order to solve this patch does:
* Set the `lld_flavor` for all Apple-derived targets to `LldFlavor::Ld64`. As far as I can see this actually works towards fixing `-Xlinker=rust-lld` as all those targets use the Mach-O object format.
* Copy/hardlink rust-lld to the gcc-ld subdirectory as ld64 next to ld.
* If `-Z gcc-ld=lld` is used and the target lld flavor is Ld64 add `-fuse-ld=/path/to/ld64` to the linker invocation.

Fixes #86945.

Forbid inline const block referencing params from being used in patterns

Fix #82518

Add Saturating type (based on Wrapping type)

Tracking #87920

### Unresolved Questions
<!--
Include any open questions that need to be answered before the feature can be
stabilised.
-->

 - [x] ~`impl Div for Saturating<T>` falls back on inner integer division - which seems alright?~
    - [x] add `saturating_div`? (to respect division by `-1`)
 - [x] There is no `::saturating_shl` and `::saturating_shr`. (How to) implement `Shl`, `ShlAssign`, `Shr` and `ShrAssign`?
   - [naively](3f7d2ce2)
 - [x] ~`saturating_neg` is only implemented on [signed integer types](https://doc.rust-lang.org/std/?search=saturating_n)~
 - [x] Is the implementation copied over from the `Wrapping`-type correct for `Saturating`?
   - [x] `Saturating::rotate_left`
   - [x] `Saturating::rotate_right`
   - [x] `Not`
   - [x] `BitXorOr` and `BitXorOrAssign`
   - [x] `BitOr` and `BitOrAssign`
   - [x] `BitAnd` and `BitAndAssign`
   - [x] `Saturating::swap_bytes`
   - [x] `Saturating::reverse_bits`

Macros 2.0-ify rustc_arena

For the purpose of battle-testing macros 2.0 I'm looking to dogfood it in rustc, one crate at a time.

(Note that there are only 12 changed lines if you ignore whitespace.)

build llvm libunwind.a in rustbuild

This PR move the building of llvm-libunwind from build script of libunwind to rustbuild, so user don't need a C compiler and recompile this C/C++ library when using build-std, and user can use codegen flags `link-self-contained` and cargo flag `build-std-features` to control the behavior of libunwind linking.