Add comments to ignore StyleCop warnings

[bfriesen]

The StyleCop analyzers pick up on the files from PolyShim and flag a bunch of warnings. To reproduce, add the following <ItemGroup> to a csproj file:

<PackageReference Include="PolyShim" Version="1.14.0">
  <PrivateAssets>all</PrivateAssets>
  <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
</PackageReference>
<PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.556">
  <PrivateAssets>all</PrivateAssets>
  <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
</PackageReference>

In order to ignore a file, StyleCop needs a "header comment" containing the sub-string "<auto-generated/>". This pull request adds that header comment to all generated files.

[Tyrrrz]

Hi. Thanks for the PR. Can you please replace This file was generated by the PolyShim library. with The following comment is required to instruct analyzers to skip this file?

[bfriesen]

Done.

[Tyrrrz]

Thank you