This ensures that each build will support the testing design of "dry
running" builds. It's also checked that a dry run build is equivalent
step-wise to a "wet" run build; the graphs we generate when running are
directly compared node/node and edge/edge, both for order and contents.

In order to run tests, previous commits have cfg'd out various parts of
rustbuild. Generally speaking, these are filesystem-related operations
and process-spawning related parts. Then, rustbuild is run "as normal"
and the various steps that where run are retrieved from the cache and
checked against the expected results.

Note that this means that the current implementation primarily tests
"what" we build, but doesn't actually test that what we build *will*
build. In other words, it doesn't do any form of dependency verification
for any crate. This is possible to implement, but is considered future
work.

This implementation strives to cfg out as little code as possible; it
also does not currently test anywhere near all of rustbuild. The current
tests are also not checked for "correctness," rather, they simply
represent what we do as of this commit, which may be wrong.

Test cases are drawn from the old implementation of rustbuild, though
the expected results may vary.

Fixes #49215

This commit is an implementation of adding custom sections to wasm artifacts in
rustc. The intention here is to expose the ability of the wasm binary format to
contain custom sections with arbitrary user-defined data. Currently neither our
version of LLVM nor LLD supports this so the implementation is currently custom
to rustc itself.

The implementation here is to attach a `#[wasm_custom_section = "foo"]`
attribute to any `const` which has a type like `[u8; N]`. Other types of
constants aren't supported yet but may be added one day! This should hopefully
be enough to get off the ground with *some* custom section support.

The current semantics are that any constant tagged with `#[wasm_custom_section]`
section will be *appended* to the corresponding section in the final output wasm
artifact (and this affects dependencies linked in as well, not just the final
crate). This means that whatever is interpreting the contents must be able to
interpret binary-concatenated sections (or each constant needs to be in its own
custom section).

To test this change the existing `run-make` test suite was moved to a
`run-make-fulldeps` folder and a new `run-make` test suite was added which
applies to all targets by default. This test suite currently only has one test
which only runs for the wasm target (using a node.js script to use `WebAssembly`
in JS to parse the wasm output).

This commit tweaks where timing and step information is printed out as part of
the build, ensuring that we do it as close to the location where work happens as
possible. In rustbuild various functions may perform long blocking work as
dependencies are assembled, so if we print out timing information early on we
may accidentally time more than just the step we were intending to time!

For LLD integration the path to `llvm-config` needed to change to inside the
build directory itself (for whatever reason) but the build directory is
different on MSBuild than it is on `ninja` for MSVC builds, so the path to
`llvm-config.exe` was actually wrong and not working!

This commit removes the `Build::llvm_config` function in favor of the source of
truth, the `Llvm` build step itself. The build step was then updated to find the
right build directory for MSBuild as well as `ninja` for where `llvm-config.exe`
is located.

Closes #48749

All uses are replaced with not accessing run.target/run.host, and
instead directly using run.builder.build.build.

This commit imports the LLD project from LLVM to serve as the default linker for
the `wasm32-unknown-unknown` target. The `binaryen` submoule is consequently
removed along with "binaryen linker" support in rustc.

Moving to LLD brings with it a number of benefits for wasm code:

* LLD is itself an actual linker, so there's no need to compile all wasm code
  with LTO any more. As a result builds should be *much* speedier as LTO is no
  longer forcibly enabled for all builds of the wasm target.
* LLD is quickly becoming an "official solution" for linking wasm code together.
  This, I believe at least, is intended to be the main supported linker for
  native code and wasm moving forward. Picking up support early on should help
  ensure that we can help LLD identify bugs and otherwise prove that it works
  great for all our use cases!
* Improvements to the wasm toolchain are currently primarily focused around LLVM
  and LLD (from what I can tell at least), so it's in general much better to be
  on this bandwagon for bugfixes and new features.
* Historical "hacks" like `wasm-gc` will soon no longer be necessary, LLD
  will [natively implement][gc] `--gc-sections` (better than `wasm-gc`!) which
  means a postprocessor is no longer needed to show off Rust's "small wasm
  binary size".

LLD is added in a pretty standard way to rustc right now. A new rustbuild target
was defined for building LLD, and this is executed when a compiler's sysroot is
being assembled. LLD is compiled against the LLVM that we've got in tree, which
means we're currently on the `release_60` branch, but this may get upgraded in
the near future!

LLD is placed into rustc's sysroot in a `bin` directory. This is similar to
where `gcc.exe` can be found on Windows. This directory is automatically added
to `PATH` whenever rustc executes the linker, allowing us to define a `WasmLd`
linker which implements the interface that `wasm-ld`, LLD's frontend, expects.

Like Emscripten the LLD target is currently only enabled for Tier 1 platforms,
notably OSX/Windows/Linux, and will need to be installed manually for compiling
to wasm on other platforms. LLD is by default turned off in rustbuild, and
requires a `config.toml` option to be enabled to turn it on.

Finally the unstable `#![wasm_import_memory]` attribute was also removed as LLD
has a native option for controlling this.

[gc]: https://reviews.llvm.org/D42511

The *.md at the root directory in src/doc are no longer tested, but this
should be fine since all files there are deprecated.

Previously, a Step would be able to tell on its own when it was invoked
"by-default" (that is, `./x.py test` was called instead of `./x.py test
some/path`). This commit replaces that functionality, invoking each Step
with each of the paths it has specified as "should be invoked by."

For example, if a step calls `path("src/tools/cargo")` and
`path("src/doc/cargo")` then it's make_run will be called twice, with
"src/tools/cargo" and "src/doc/cargo." This makes it so that default
handling logic is in builder, instead of spread across various Steps.

However, this meant that some Step specifications needed to be updated,
since for example `rustdoc` can be built by `./x.py build
src/librustdoc` or `./x.py build src/tools/rustdoc`. A `PathSet`
abstraction is added that handles this: now, each Step can not only list
`path(...)` but also `paths(&[a, b, ...])` which will make it so that we
don't invoke it with each of the individual paths, instead invoking it
with the first path in the list (though this shouldn't be depended on).

Future work likely consists of implementing a better/easier way for a
given Step to work with "any" crate in-tree, especially those that want
to run tests, build, or check crates in the std, test, or rustc crate
trees. Currently this is rather painful to do as most of the logic is
duplicated across should_run and make_run. It seems likely this can be
abstracted away into builder somehow.

Building on the work of # 45684 this commit updates the compiler to
unconditionally load the `rustc_trans` crate at runtime instead of linking to it
at compile time. The end goal of this work is to implement # 46819 where rustc
will have multiple backends available to it to load.

This commit starts off by removing the `extern crate rustc_trans` from the
driver. This involved moving some miscellaneous functionality into the
`TransCrate` trait and also required an implementation of how to locate and load
the trans backend. This ended up being a little tricky because the sysroot isn't
always the right location (for example `--sysroot` arguments) so some extra code
was added as well to probe a directory relative to the current dll (the
rustc_driver dll).

Rustbuild has been updated accordingly as well to have a separate compilation
invocation for the `rustc_trans` crate and assembly it accordingly into the
sysroot. Finally, the distribution logic for the `rustc` package was also
updated to slurp up the trans backends folder.

A number of assorted fallout changes were included here as well to ensure tests
pass and such, and they should all be commented inline.

These no longer work now that Cargo changes the cwd of rustc while it's running.
Instead use an absolute path that's set by rustbuild.

This reverts commit ab018c76.

This also adds the `ToolBuild::is_ext_tool` field to replace the previous
`ToolBuild::expectation` field, to indicate whether a build-failure of
certain tool is essential.

This commit allocates a builder to running wasm32 tests on Travis. Not all test
suites pass right now so this is starting out with just the run-pass and the
libcore test suites. This'll hopefully give us a pretty broad set of coverage
for integration in rustc itself as well as a somewhat broad coverage of the llvm
backend itself through integration/unit tests.