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ARROW-5149: [Packaging][Wheel] Pin LLVM to version 7 in windows builds

Crossbow builds: https://github.com/kszucs/crossbow/branches/all?query=build-549

Author: Krisztián Szűcs <szucs.krisztian@gmail.com>

Closes #4130 from kszucs/win-wheel-pin-llvm and squashes the following commits:

3a02e8466 <Krisztián Szűcs> pin llvm=7