The Windows installer fails when running unelevated

[tristanlabelle]

Since #251, the installer packages the VC redistributable, which is a machine-wide install, even though the Swift toolchain itself if a per-user install. This step hence fails when running the installer unelevated. We might be able to switch back to using ExePackage, which I believe will show an elevation prompt but has other limitations on detecting prior installations, or we could mark that step Vital="false" and allow it to fail when run unelevated.

[barnson]

Burn uses CreateProcessW to launch ExePackages too, so there's no UAC prompt.

[tristanlabelle]

Fixed by bundling individual dlls via swiftlang/swift#69743 and #256