It has no effect on deployment error and may cause further network issues.

Currently on CI we predominately compile LLVM with the default system compiler
which means gcc on Linux, some version of Clang on OSX, MSVC on Windows, and
gcc on MinGW. This commit switches Linux, OSX, and Windows to all use Clang
6.0.0 to build LLVM (aka the C/C++ compiler as part of the bootstrap). This
looks to generate faster code according to #49879 which translates to a faster
rustc (as LLVM internally is faster)

The major changes here were to the containers that build Linux releases,
basically adding a new step that uses the previous gcc 4.8 compiler to compile
the next Clang 6.0.0 compiler. Otherwise the OSX and Windows scripts have been
updated to download precompiled versions of Clang 6 and configure the build to
use them.

Note that `cc` was updated here to fix using `clang-cl` with `cc-rs` on MSVC, as
well as an update to `sccache` on Windows which was needed to correctly work
with `clang-cl`. Finally the MinGW compiler is entirely left out here
intentionally as it's currently thought that Clang can't generate C++ code for
MinGW and we need to use gcc, but this should be verified eventually.

Bots that read the log can simply look for `[CI_JOB_NAME=...]` to find out
the job's name.

This builder is starting to time out frequently causing PRs to bounce and
otherwise doesn't seem to be catching too many bugs, so this commit removes it
entirely. We've had a number of timeouts in the last few weeks related to this
builder:

* https://travis-ci.org/rust-lang/rust/jobs/360947582
* https://travis-ci.org/rust-lang/rust/jobs/360464190
* https://travis-ci.org/rust-lang/rust/jobs/359946975
* https://travis-ci.org/rust-lang/rust/jobs/361213241
* https://travis-ci.org/rust-lang/rust/jobs/362346279
* https://travis-ci.org/rust-lang/rust/jobs/362072331

On a good run this builder takes about 2h15m, which is already too long for
Travis and the variable build times end up pushing it beyond the 3h limit
occasionally.

The timeouts here are somewhat expected in that an incrementally compiled rustc
compiler isn't optimized like a normal rustc, disallowing inlining between
codegen units and losing lots of optimization opportunities.

Ideally I'd like to soon enable sccache for rustbuild itself and some of the
stage0 tools, but for that to work we'll need some better Rust support than the
pretty old version we were previously using!

This commit moves away from caching on Travis to our own caching on S3 for
caching docker layers between builds. Unfortunately the Travis caches have over
time had a few critical pain points:

* Caches are only updated for successful builds, meaning that if a build times
  out or fails in a different location the sucessfully-created docker images
  isn't always cached. While this makes sense as a general rule of caches it
  hurts our use cases.

* Caches are per-branch and builder which means that we don't have a separate
  cache on each release channel. All our merges go through the `auto` branch
  which means that they're all sharing the same cache, even those for merging to
  master/beta. This means that PRs which switch between master/beta will keep
  rebuilting and having cache misses.

* Caches have historically been invaliated somewhat regularly a little more
  aggressively than we'd want (I think).

* We don't always need to update the contents of the cache if the Docker image
  didn't change at all, and saving off the docker layers can sometimes be quite
  expensive.

For all these reasons this commit drops the usage of Travis's built-in caching
support. Instead our own caching is used by storing blobs to S3. Normally this
would be a very risky endeavour but we're basically priming a cache for a cache
(docker) so if we get this wrong the failure mode is longer builds, not stale
caches. We'll notice that pretty quickly and hopefully fix it!

The logic here is inserted directly into the `src/ci/docker/run.sh` script to
download an image based on a shasum of the `Dockerfile` and other assorted files.
This blob, if found, is loaded into docker and we record what layers were
inserted. After docker finishes the build (hopefully quickly with lots of cache
hits) we then see the sha of the final image. If it's one of the layers we
loaded then there's no need to update the cache. Otherwise we upload our layers
to the global cache, possibly overwriting what we previously just downloaded.

This is hopefully a step towards mitigating #49278 although it doesn't
completely fix it as it means we'll still probably have to retry builds that
bust the cache.

This reverts commit 20e65f11.

This commit upgrades the dist builders for OSX to Travis's new `xcode9.3-moar`
image which has 3 cores available to it instead of 2. This should help us
provide speedier builds on OSX and hit timeouts less in theory!

Note that historically the dist builders for OSX have been a different version
than the ones that are running tests. I had forgotten why this was the case and
digging around brought up 30761556 where apparently Xcode 8 wasn't able to
compile LLVM with `MACOSX_DEPLOYMENT_TARGET=10.7` which we desired. On a whim I
gave this PR a spin and it [looks like][green] this has since been fixed (maybe
in LLVM?). In any case those green builds should hopefully mean that we can
safely upgrade and get faster infrastructure to boot.

This commit also includes an upgrade of OpenSSL. This is not done for security
reasons but rather build system reasons. Originally builds with the new image
[did not succeed][red] due to weird build failures in OpenSSL, but upgrading
seems to have made the spurious errors go away to here's to also hoping that's
fixed!

[green]: https://travis-ci.org/rust-lang/rust/builds/351353412
[red]: https://travis-ci.org/rust-lang/rust/builds/350969248

This upgrades the OSX builders to the `xcode9.3-moar` image which has 3 cores as
opposed to the 2 that our builders currently have. Should help make those OSX
builds a bit speedier!

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

Fix rust-lang-nursery/rust-toolstate#2.

This commit introduces a separately compiled backend for Emscripten, avoiding
compiling the `JSBackend` target in the main LLVM codegen backend. This builds
on the foundation provided by #47671 to create a new codegen backend dedicated
solely to Emscripten, removing the `JSBackend` of the main codegen backend in
the process.

A new field was added to each target for this commit which specifies the backend
to use for translation, the default being `llvm` which is the main backend that
we use. The Emscripten targets specify an `emscripten` backend instead of the
main `llvm` one.

There's a whole bunch of consequences of this change, but I'll try to enumerate
them here:

* A *second* LLVM submodule was added in this commit. The main LLVM submodule
  will soon start to drift from the Emscripten submodule, but currently they're
  both at the same revision.
* Logic was added to rustbuild to *not* build the Emscripten backend by default.
  This is gated behind a `--enable-emscripten` flag to the configure script. By
  default users should neither check out the emscripten submodule nor compile
  it.
* The `init_repo.sh` script was updated to fetch the Emscripten submodule from
  GitHub the same way we do the main LLVM submodule (a tarball fetch).
* The Emscripten backend, turned off by default, is still turned on for a number
  of targets on CI. We'll only be shipping an Emscripten backend with Tier 1
  platforms, though. All cross-compiled platforms will not be receiving an
  Emscripten backend yet.

This commit means that when you download the `rustc` package in Rustup for Tier
1 platforms you'll be receiving two trans backends, one for Emscripten and one
that's the general LLVM backend. If you never compile for Emscripten you'll
never use the Emscripten backend, so we may update this one day to only download
the Emscripten backend when you add the Emscripten target. For now though it's
just an extra 10MB gzip'd.

Closes #46819

There is not enough capacity to do automated builds for CloudABI at this
time.

Setting up a cross compilation toolchain for CloudABI is relatively
easy. It's just a matter of installing a somewhat recent version of
Clang (5.0 preferred) and installing the corresponding
${target}-cxx-runtime package, containing a set of core C/C++ libraries
(libc, libc++, libunwind, etc).

Eventually it would be nice if we could also run 'x.py test'. That,
however still requires some more work. Both libtest and compiletest
would need to be adjusted to deal with CloudABI's requirement of having
all of an application's dependencies injected. Let's settle for just
doing 'x.py dist' for now.

If a PR intends to update a tool but its test has failed, abort the merge
regardless of current channel. This should help the tool maintainers if the
update turns out to be failing due to changes in latest master.

It seems using `fe80::/64` causes `docker start` to fail with "Address
already in use". Try to change to a unique local address range instead.

This reverts commit c0c26a64.

Use it until travis-ci/travis-ci#8891 is fixed.

Follows the convention of the other builders.

The cargotest job is renamed to tools for clarification.