[master] Update dependencies from 4 repositories (dotnet/corefx#41424)

[master] Update dependencies from 4 repositories (dotnet/corefx#41424)

* Update dependencies from https://github.com/dotnet/arcade build 20190927.2

- Microsoft.DotNet.XUnitExtensions - 5.0.0-beta.19477.2
- Microsoft.DotNet.XUnitConsoleRunner - 2.5.1-beta.19477.2
- Microsoft.DotNet.VersionTools.Tasks - 5.0.0-beta.19477.2
- Microsoft.DotNet.ApiCompat - 5.0.0-beta.19477.2
- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19477.2
- Microsoft.DotNet.Build.Tasks.Configuration - 5.0.0-beta.19477.2
- Microsoft.DotNet.Build.Tasks.Feed - 5.0.0-beta.19477.2
- Microsoft.DotNet.Build.Tasks.Packaging - 5.0.0-beta.19477.2
- Microsoft.DotNet.CodeAnalysis - 5.0.0-beta.19477.2
- Microsoft.DotNet.CoreFxTesting - 5.0.0-beta.19477.2
- Microsoft.DotNet.GenAPI - 5.0.0-beta.19477.2
- Microsoft.DotNet.GenFacades - 5.0.0-beta.19477.2
- Microsoft.DotNet.Helix.Sdk - 5.0.0-beta.19477.2
- Microsoft.DotNet.RemoteExecutor - 5.0.0-beta.19477.2

* Update dependencies from https://github.com/dotnet/core-setup build 20190927.25

- Microsoft.NETCore.App - 5.0.0-alpha1.19477.25
- Microsoft.NETCore.DotNetHostPolicy - 5.0.0-alpha1.19477.25
- Microsoft.NETCore.DotNetHost - 5.0.0-alpha1.19477.25

* Update dependencies from https://github.com/dotnet/corefx build 20190927.7

- runtime.native.System.IO.Ports - 5.0.0-alpha1.19477.7
- Microsoft.NETCore.Platforms - 5.0.0-alpha1.19477.7

* Update dependencies from https://github.com/dotnet/standard build 20190927.1

- NETStandard.Library - 2.2.0-prerelease.19477.1

* Update dependencies from https://github.com/dotnet/core-setup build 20190928.4

- Microsoft.NETCore.App - 5.0.0-alpha1.19478.4
- Microsoft.NETCore.DotNetHostPolicy - 5.0.0-alpha1.19478.4
- Microsoft.NETCore.DotNetHost - 5.0.0-alpha1.19478.4

* Update dependencies from https://github.com/dotnet/standard build 20190928.2

- NETStandard.Library - 2.2.0-prerelease.19478.2

* Update dependencies from https://github.com/dotnet/core-setup build 20190929.2

- Microsoft.NETCore.App - 5.0.0-alpha1.19479.2
- Microsoft.NETCore.DotNetHostPolicy - 5.0.0-alpha1.19479.2
- Microsoft.NETCore.DotNetHost - 5.0.0-alpha1.19479.2

* Enable nullable in source projects that are full facades and generate a
not supported assembly with annotations in it

Commit migrated from https://github.com/dotnet/corefx/commit/4eece5734d828ee8bf3393a04b71d80d5d5e4828