The session-derive-errors test ensures the internal SessionDiagnostic
derive macro outputs the right error messages when misused.

The macro relies on the proc_macro2 crate though, which changes its span
behavior depending on whether the channel is nightly or not. This caused
test failures when bumping the channel from nightly to beta/stable.

Since SessionDiagnostic is internal-only we don't care about its
diagnostics quality outside of nightly, as the compiler itself is
developed on nightly. Thus the easiest solution is to ignore that test
on the beta and stable channels.

This also implements `// only-{channel}` and `// ignore-{channel}` in
compiletest to properly support the change.

- Pass it explicitly where appropriate
- Update stderr files and warnings; it turns that unstable-options has
  far-reaching effects on diagnostics.

 ## User-facing changes

- Intra-doc links to primitives that currently go to rust-lang.org/nightly/std/primitive.x.html will start going to channel that rustdoc was built with. Nightly will continue going to /nightly; Beta will link to /beta; stable compilers will link to /1.52.1 (or whatever version they were built as).
- Cross-crate links from std to core currently go to /nightly unconditionally. They will start going to /1.52.0 on stable channels (but remain the same on nightly channels).
- Intra-crate links from std to std (or core to core) currently go to the same URL they are hosted at; they will continue to do so. Notably, this is different from everything else because it can preserve the distinction between /stable and /1.52.0 by using relative links.

Note that "links" includes both intra-doc links and rustdoc's own
automatically generated hyperlinks.

 ## Implementation changes

- Update the testsuite to allow linking to /beta and /1.52.1 in docs
- Use an html_root_url for the standard library that's dependent on the channel

  This avoids linking to nightly docs on stable.

- Update rustdoc to use channel-dependent links for primitives from an
  unknown crate

- Set DOC_RUST_LANG_ORG_CHANNEL from bootstrap to ensure it's in sync
- Include doc.rust-lang.org in the channel

- Add rustfmt to `x.py check`
- Update Cargo.lock
- Remove rustfmt from the toolstate list
- Make rustfmt an in-tree tool
- Give an error on `x.py test rustfmt` if rustfmt fails to build or if tests fail
- Don't call `save_toolstate` when testing rustfmt

MinGW driver for COFF LLD doesn't currently translate GNU-style `--threads=N` to native `/threads:N`, so we have to pass the option in its native form to avoid an error.

Also pass the `threads` flag to lld-link as well

This allows running a single test without having to wait for all tests
to complete.

- Add clippy_dev to the rust workspace

  Before, it would give an error that it wasn't either included or
  excluded from the workspace:

  ```
  error: current package believes it's in a workspace when it's not:
  current:   /home/joshua/rustc/src/tools/clippy/clippy_dev/Cargo.toml
  workspace: /home/joshua/rustc/Cargo.toml

  this may be fixable by adding `src/tools/clippy/clippy_dev` to the `workspace.members` array of the manifest located at: /home/joshua/rustc/Cargo.toml
  Alternatively, to keep it out of the workspace, add the package to the `workspace.exclude` array, or add an empty `[workspace]` table to the package's manifest.
  ```

- Change clippy's copy of compiletest not to special-case
  rust-lang/rust. Using OUT_DIR confused `clippy_dev` and it couldn't find
  the test outputs. This is one of the reasons why `cargo dev bless` used
  to silently do nothing (the others were that `CARGO_TARGET_DIR` and
  `PROFILE` weren't set appropriately).

- Run clippy_dev on test failure

I tested this by removing a couple lines from a stderr file, and they
were correctly replaced.

- Fix clippy_dev warnings

the test harness already spawns enough tests for all cores, individual
tests should keep their own threading to a minimum to avoid context switch
overhead

some tests fail with 1 CGU, so explicit compile flags have been added
to keep their old behavior

Adds bootstrap rules to support installing rust-demangler.

When compiling with `-Z instrument-coverage`, the coverage reports are
generated by `llvm-cov`. `llvm-cov` includes a built-in demangler for
C++, and an option to supply an alternate demangler. For Rust, we have
`rust-demangler`, currently used in `rustc` coverage tests.

Fuchsia's toolchain for Rust is built via `./x.py install`. Fuchsia is
adding support for Rust coverage, and we need to include the
`rust-demangler` in the installed `bin` directory.

Configured rust-demangler as an in-tree extended tool.

Added tests to support `./x.py test rust-demangler`.

Install with extended tools by default only if `profiler = true`.

We don't need to complain to the user about linkcheck having different
hosts and targets when it is already excluded. This can be achieved by
moving the check to when the step is run instead of in should_run.

When we cross compile, some things (and their documentation) are built
for the host (e.g. rustc), while others (and their documentation) are built
for the target. This generated documentation will have broken links
between documentation for different platforms e.g. between rustc and
cargo.

To be clear, by default running `x.py test` on a checkout of the beta branch
currently fails, and with this change will continue to fail, because `x.py
tests` runs `x.py test src/tools/tidy` which tries to run `rustfmt` and that
will fail because the `rustfmt` binary is pinned to the current nighlty and we
do not attempt to distribute one for the beta builds.

This change gives a better error message than the current message, which is just
"./x.py fmt is not supported on this channel" without providing any hint about
what one might do about that problem.

(update: placated tidy.)

Rustdoc almost never needs a full stage 2 compiler, and requiring
rustdoc tests to be in run-make-fulldeps adds a lot of compile time for
no reason.

Previously, there two different ways to encode the same info: `None` or
`Some(&[])`. Now there is only one way, `&[]`.

Newly added targets aren't available on the stage0 toolchain.

The struct will allow to store more context on the generated tarballs.

addresses:

clippy::or_fun_call
clippy::single_char_add_str
clippy::comparison_to_empty
clippy::or_fun_call