* 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`
ab7f3f2018