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

renesugar/arrow,kou/arrow,cpcloud/arrow,cpcloud/arrow,renesugar/arrow,xhochy/arrow,cpcloud/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,renesugar/arrow,majetideepak/arrow,xhochy/arrow,kou/arrow,apache/arrow,kou/arrow,icexelloss/arrow,kou/arrow,laurentgo/arrow,icexelloss/arrow,kou/arrow,majetideepak/arrow,majetideepak/arrow,apache/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,xhochy/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,wesm/arrow,kou/arrow,icexelloss/arrow,renesugar/arrow,apache/arrow,icexelloss/arrow,apache/arrow,cpcloud/arrow,xhochy/arrow,wesm/arrow,kou/arrow,renesugar/arrow,xhochy/arrow,xhochy/arrow,laurentgo/arrow,cpcloud/arrow,wesm/arrow,icexelloss/arrow,xhochy/arrow,renesugar/arrow,majetideepak/arrow,cpcloud/arrow,wesm/arrow,laurentgo/arrow,xhochy/arrow,icexelloss/arrow,cpcloud/arrow,apache/arrow,renesugar/arrow,majetideepak/arrow,laurentgo/arrow,laurentgo/arrow,cpcloud/arrow,cpcloud/arrow,apache/arrow,majetideepak/arrow,xhochy/arrow,apache/arrow,majetideepak/arrow,renesugar/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,majetideepak/arrow,renesugar/arrow,apache/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,renesugar/arrow,icexelloss/arrow,cpcloud/arrow,majetideepak/arrow,cpcloud/arrow,renesugar/arrow,apache/arrow,laurentgo/arrow,majetideepak/arrow,wesm/arrow,laurentgo/arrow,xhochy/arrow,kou/arrow,cpcloud/arrow,wesm/arrow,icexelloss/arrow,wesm/arrow,majetideepak/arrow,wesm/arrow,xhochy/arrow,xhochy/arrow,icexelloss/arrow,laurentgo/arrow,renesugar/arrow,kou/arrow,majetideepak/arrow,majetideepak/arrow,xhochy/arrow,majetideepak/arrow,renesugar/arrow,apache/arrow,apache/arrow,renesugar/arrow,laurentgo/arrow,kou/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,majetideepak/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,xhochy/arrow,wesm/arrow,xhochy/arrow