WiX: use Swift logo for the installer

This provides us with a nicer non-pixelated logo for the installer.  The
logo is sourced from https://developer.apple.com/swift/resources/ with
the SVG converted to PNG at 256x256@96.

Author: compnerd

platforms/Windows/bundle/installer.wxs

@@ -2,6 +2,7 @@
   <Bundle
     Name="!(loc.BundleName) $(ProductVersion)"
     Version="$(ProductVersion)"
+    IconSourceFile="swift_logo.png"
     Manufacturer="!(loc.ManufacturerName)"
     Compressed="$(IsBundleCompressed)"
     UpgradeCode="$(BundleUpgradeCode)">