This bundles clang/lld and associated tools, configured for a mingw
target, with corresponding mingw sysroots.
As we already install the 'llvm' chocolatey package, which also
provides clang (defaulting to a MSVC target), add this one last in the
path. This lets 'clang' refer to the default MSVC-targeting tool, while
one can use a GCC style triple prefixed driver name, like
x86_64-w64-mingw32-clang, to use this tool.
This allows setting up a much-needed mingw configuration for the libcxx
CI.
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