This commit adds support for sccache, a ccache-like compiler which works on MSVC
and stores results into an S3 bucket. This also switches over all Travis and
AppVeyor automation to using sccache to ensure a shared and unified cache over
time which can be shared across builders.
The support for sccache manifests as a new `--enable-sccache` option which
instructs us to configure LLVM differently to use a 'sccache' binary instead of
a 'ccache' binary. All docker images for Travis builds are updated to download
Mozilla's tooltool builds of sccache onto various containers and systems.
Additionally a new `rust-lang-ci-sccache` bucket is configured to hold all of
our ccache goodies.
96a5fc76dc