Clean up debugging options

I found various sub-optimal things when I was looking at option handling.

Rollup of 7 pull requests

Successful merges:

 - #70331 (Increase verbosity when using update syntax with private fields)
 - #70349 (move `hir_id_validation` to `rustc_passes` + simplify `hir::map` code)
 - #70361 (Update backtrace crate to 0.3.46)
 - #70364 (resolve: Remove `rustc_attrs` as a standalone feature gate)
 - #70369 (Fix smaller issues with invalid placeholder type errors)
 - #70373 (normalize some imports & prefer direct ones)
 - #70376 (Add test for #66312)

Failed merges:

r? @ghost

Add test for #66312

Closes #66312. This issue was fixed by #68884.

r? @Zoxc

normalize some imports & prefer direct ones

r? @Mark-Simulacrum

Fix smaller issues with invalid placeholder type errors

Follow up to #70294.

- Fix placement of suggested generic param when bounds are present.
- Reduce error duplication for invalid placeholder types in `fn` types.

r? @Centril

resolve: Remove `rustc_attrs` as a standalone feature gate

Now it only gates specific built-in attributes.

So if you want to make a rustc attribute, make it a built-in (this was already the case in practice for some time).

Update backtrace crate to 0.3.46

* Support line-tables-only when using libbacktrace
* Update libbacktrace to latest master
* Define HAVE_KERN_PROC on FreeBSD to fix rust-lang/rust#54434

move `hir_id_validation` to `rustc_passes` + simplify `hir::map` code

r? @Zoxc

Increase verbosity when using update syntax with private fields

Resolves #70323

Closes #66312. This issue was fixed by #68884.

Rollup of 8 pull requests

Successful merges:

 - #70023 (clean up E0436 explanation)
 - #70234 (#[track_caller] on core::ops::{Index, IndexMut}.)
 - #70241 (Miri: move ModifiedStatic to ConstEval errors)
 - #70342 (IoSlice/IoSliceMut should be Send and Sync)
 - #70350 (Request "-Z unstable-options" for unstable options)
 - #70355 (Clean up E0454)
 - #70359 (must_use on split_off)
 - #70368 (Mark hotplug_codegen_backend as ignore-stage1)

Failed merges:

r? @ghost

Mark hotplug_codegen_backend as ignore-stage1

Suggested by @EddyB in https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/do.20we.20really.20need.20stage2.2E.2E.3F/near/191659846.

must_use on split_off

A couple more for #70194

Clean up E0454

r? @Dylan-DPC

Request "-Z unstable-options" for unstable options

Explicitly requests the "-Z unstable-options" flag if someone attempts to use
a cargo option gated by it. This enhances discoverability, particularly in the
instance where the user is on the nightly compiler but isn't using the flag.

This relates to, but does not end with or resolve, issue #65770.

IoSlice/IoSliceMut should be Send and Sync

Closes #70308

Miri: move ModifiedStatic to ConstEval errors

Also generally adjust terminology from "static" to "global" where appropriate (to avoid confusion with specifically `static` items).

#[track_caller] on core::ops::{Index, IndexMut}.

Applies the attribute to `core::ops::Index(Mut)` and enough std internals to cover the [functions listed in "tier 1" in the original RFC](https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md#survey-of-panicking-standard-functions).

Split out from #69251 to allow separate assessment of perf impact.

To my knowledge, this is the last piece of implementing RFC 2091.

Tracking issue: https://github.com/rust-lang/rust/issues/47809

clean up E0436 explanation

r? @Dylan-DPC

Now it only gates specific built-in attributes

#70194

Add GitHub Actions configuration

This PR adds the GitHub Actions configuration to the rust-lang/rust repository. The configuration will be run in parallel with Azure Pipelines until the evaluation finishes: the infrastructure team will then decide whether to switch.

Since GitHub Actions doesn't currently have any way to include pieces of configuration, this also adds the `src/tools/expand-yaml-anchors` tool, which serves as a sort of templating system. Otherwise the configuration is a mostly straight port from the Azure Pipelines configuration (thanks to all the PRs opened in the past).

There are still a few small things I need to fix before we can land this, but it's mostly complete and ready for an initial review.

r? @Mark-Simulacrum

This will inform contributors tweaking the Azure Pipelines configuration
that they also need to tweak the GitHub Actions setup.

Moving just the `obj` directory created problems with mountpoints and
Docker containers, so this tries to symlink the parent directory.

The /mnt mount point has 53GB of free disk space at the time of writing
this commit, so this moves the build there to avoid running out of disk
space during builds.

On Azure Pipeliones, the C: filesystem is huge with a lot of free space,
while D: is small. By default builds happened in D:, so we added a
script to symlink the big directories to C:, granting us more space.

    Filesystem  Size  Used Avail Use%
    C:          256G  143G  114G  56%
    D:           14G  2.0G   13G  15%

On GitHub Actions instead C: is almost full, and we have a lot of free
space on D:, where the build happens.

    Filesystem  Size  Used Avail Use%
    C:          128G  114G   15G  89%
    D:          56G   4.8G   52G   9%

This commit stops creating the symlink on GitHub Actions, fixing the out
of disk space errors we were seeing on some Windows builders.

There will be a need to symlink the Linux build directory in the future
as well, so let's make the script name generic.