* Enable `/warnAsError` in Windows builds
  - already enabled in non-Windows builds because override existed only in build.ps1
* Allow some warnings related to closed issues
  - common `<NoWarn>$(NoWarn);CS1591</NoWarn>` case unchanged
    - /Directory.Build.props ensures that warning remains a warning but doesn't hide it
* !fixup! Root build.sh _does_ disable warnings as errors
  - variable eng/common/tools.sh uses named `warn_as_error`