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.