ci: Compile LLVM with Clang 6.0.0
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.
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