Feature: Enable setting the "NoWarn" option on a per-file basis

[rainersigwald]

From @fubar-coder on January 21, 2019 13:57

Steps to reproduce

Example project file (as suggestion):

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp2.2</TargetFramework>
    <GenerateDocumentationFile Condition="'$(Configuration)' == 'Release'">true</GenerateDocumentationFile>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.App" />
    <PackageReference Include="Microsoft.AspNetCore.Razor.Design" Version="2.2.0" PrivateAssets="All" />
  </ItemGroup>
  <ItemGroup>
    <Compile Update="Program.cs">
      <NoWarn>CS1591;$(NoWarn)</NoWarn>
    </Compile>
  </ItemGroup>
</Project>

Expected behavior

No CS1591 warning for Program.cs.

Actual behavior

I get a warning for Program.cs.

Copied from original issue: dotnet/msbuild#4087

[rainersigwald]

This is a C#-compiler feature request, so I'll move it to the Roslyn repo.