build: flesh out the Windows toolchain build
build: flesh out the Windows toolchain build

This adds a complete toolchain bootstrap script for Windows, building
the entire toolchain, standard libraries, Swift Package Manager, and
SourceKit-LSP.  Additionally, it packages the resulting binaries into
the appropriate MSIs and builds the installer.

xwu/swift,apple/swift,xwu/swift,roambotics/swift,rudkx/swift,glessard/swift,atrick/swift,gregomni/swift,JGiola/swift,glessard/swift,gregomni/swift,apple/swift,hooman/swift,xwu/swift,apple/swift,glessard/swift,hooman/swift,glessard/swift,JGiola/swift,rudkx/swift,rudkx/swift,hooman/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,ahoppen/swift,JGiola/swift,JGiola/swift,ahoppen/swift,rudkx/swift,rudkx/swift,benlangmuir/swift,atrick/swift,ahoppen/swift,hooman/swift,JGiola/swift,apple/swift,atrick/swift,JGiola/swift,gregomni/swift,benlangmuir/swift,apple/swift,roambotics/swift,gregomni/swift,hooman/swift,benlangmuir/swift,xwu/swift,roambotics/swift,ahoppen/swift,roambotics/swift,roambotics/swift,apple/swift,hooman/swift,atrick/swift,xwu/swift,atrick/swift,hooman/swift,glessard/swift,ahoppen/swift,glessard/swift,roambotics/swift,rudkx/swift,xwu/swift,xwu/swift,gregomni/swift,gregomni/swift,ahoppen/swift,atrick/swift