Replace Rc with Lrc for shared data

This replaces `Rc`s reachable from `TyCtxt` with `Lrc`. This has no effect unless `cfg(parallel_queries)` is set. It also contains a fix for the `Decodable` impl for `Arc`.

r? @nikomatsakis

Another rollup

None

Rebased refactorings for Chalk

The code is Niko's, I just handled the rebase.

r? @nikomatsakis

Add AArch64 features to whitelist

Fix link to rustc guide in README.md

This is a follow-up to #48479 and fixes a minor bug with the link to the rustc guide in the README.

r? @nikomatsakis

cc @mark-i-m

Add regression test for #48551

[Issue link](https://github.com/rust-lang/rust/issues/48551)

Stabilize LocalKey::try_with

The `LocalKey::try_with` method is now stabilized.

`LocalKey::state` and `LocalKeyState` marked as deprecated. Although, is there any reason to keep them - should we perhaps remove them completely?

Closes #27716

r? @alexcrichton

rustc: Tweak funclet cleanups of ffi functions

This commit is targeted at addressing #48251 by specifically fixing a case where
a longjmp over Rust frames on MSVC runs cleanups, accidentally running the
"abort the program" cleanup as well. Added in #46833 `extern` ABI functions in
Rust will abort the process if Rust panics, and currently this is modeled as a
normal cleanup like all other destructors.

Unfortunately it turns out that `longjmp` on MSVC is implemented with SEH, the
same mechanism used to implement panics in Rust. This means that `longjmp` over
Rust frames will run Rust cleanups (even though we don't necessarily want it
to). Notably this means that if you `longjmp` over a Rust stack frame then that
probably means you'll abort the program because one of the cleanups will abort
the process.

After some discussion on IRC it turns out that `longjmp` doesn't run cleanups
for *caught* exceptions, it only runs cleanups for cleanup pads. Using this
information this commit tweaks the codegen for an `extern` function to
a catch-all clause for exceptions instead of a cleanup block. This catch-all is
equivalent to the C++ code:

    try {
        foo();
    } catch (...) {
        bar();
    }

and in fact our codegen here is designed to match exactly what clang emits for
that C++ code!

With this tweak a longjmp over Rust code will no longer abort the process. A
longjmp will continue to "accidentally" run Rust cleanups (destructors) on MSVC.
Other non-MSVC platforms will not rust destructors with a longjmp, so we'll
probably still recommend "don't have destructors on the stack", but in any case
this is a more surgical fix than #48567 and should help us stick to standard
personality functions a bit longer.

Fix find_width_of_character_at_span bounds check

Commit 0bd96671 added bounds checking of our current target byte position to prevent infinite loops. Unfortunately it was comparing the file-relative `target` versus the global `file_start_pos` and `file_end_pos`.

The result is failing to detect multibyte characters unless their file-relative offset fit within their global offset. This causes other parts of the compiler to generate spans pointing to the middle of a
multibyte character which will ultimately panic in `bytepos_to_file_charpos`.

Fix by comparing the `target` to the total file size when moving forward and doing checked subtraction when moving backwards. This should preserve the intent of the bounds check while removing the offset confusion.

cc @davidtwco

Fixes #48508

Support parentheses in patterns under feature gate

This is a prerequisite for any other extensions to pattern syntax - `|` with multiple patterns, type ascription, `..PAT` in slice patterns.

Closes https://github.com/rust-lang/rfcs/issues/554

Remove E0245; improve E0404

Fix #36337

Somehow this is currently breaking --explain, but I don't understand how.

r? @estebank

Auto-toolstate management follow-up.

Tracking comment: https://github.com/rust-lang/rust/issues/45861#issuecomment-367302777

* Fixed rust-lang-nursery/rust-toolstate#1, a proper link to the PR will be included.
* Fixed rust-lang-nursery/rust-toolstate#2, a comment will be posted to the PR if the toolstate changed
* Toolstate regression will be rejected at the last week of the 6-week cycle (currently entirely date-based).
* Implemented https://internals.rust-lang.org/t/the-current-submodule-setup-is-not-tenable/6593, moved doc tests of Nomicon, Reference, Rust-by-Example and The Book to the "tools" job and thus allowed to fail like other external tools.

Additional uses of this item were added to these files in #45701 and #46479

These modules were replaced with re-exports from ena

These changes were meant to be in
2b18d8fe9dc05415a8e6b7cadf879c7f7ebe020a (rebased from
12a230562ece9b0d29018a436676141054dc53b7), but I messed up the rebase a
bit as the file had been moved.

This should not be needed: the new variable will be related to the old
ones, so if they are constrained, so is the new variable; if they are
not, and hence default to diverging, so will the new variable.

The ena version has an improved interface. I suspect
`librustc_data_structures` should start migrating out to crates.io in
general.

This was all unused anyway.

Always using root environment for now.