This creates a new toolchain artifact that repacks a combination of the
linux64-clang compiler along with parts of the win64-clang-cl compiler.

This has multiple advantages:

• It removes some convoluted parts of build task definitions (limiting
  that to only occur on the win-cross toolchain itself).
• It simplifies the build setup by not requiring to prepare for where
  clang-cl.exe is.
• It speeds up getting compiler artifacts because the win64-clang-cl
  artifact is very large (due to there not being a llvm shared library)
  and bzipped, which is slow to decompress. Here, we only take what we
  need for the cross builds.
• It adds the runtime files that e.g. PGO will require, and that
  linux clang-cl insists lives in the clang directory, not the
  win64-clang-cl one, and that would require some convoluted setup to make
  it work with the two separate toolchains.

https://hg.mozilla.org/mozilla-central/rev/8937070f0f36