Build intermittently fails due to missing generated source files

* .NET Core Version: Version: 3.0.0-preview7-27902-19
* Windows version: 18362.239

**Problem description:**
We've been intermittently hitting build failures in recent builds due to files usually generated by the XAML compiler missing. It will fail one time and immediately succeed next build and then won't fail against for a couple of hours. Multiple folks have started hitting it.

I happened to capture a failure under a build log.

**Actual behavior:**
```
CSC : error CS2001: Source file 'C:\project-system\artifacts\Debug\obj\Microsoft.VisualStudio.ProjectSystem.Managed.VS\ProjectSystem\VS\PropertyPages\GetProfileNameDialog.g.cs' could not be found. [C:\project-system\src\Microsoft.VisualStudio.ProjectSystem.Managed.VS\Microsoft.VisualStudio.ProjectSystem.Managed.VS_doj0ahs0_wpftmp.csproj]
CSC : error CS2001: Source file 'C:\project-system\artifacts\Debug\obj\Microsoft.VisualStudio.ProjectSystem.Managed.VS\ ProjectSystem\VS\PropertyPages\DebugPageControl.g.cs' could not be found. [C:\project-system\src\Microsoft.VisualStudio .ProjectSystem.Managed.VS\Microsoft.VisualStudio.ProjectSystem.Managed.VS_doj0ahs0_wpftmp.csproj]
```
**Expected behavior:**
No build failures.

**Minimal repro:**
Build Log: [Build.zip](https://github.com/dotnet/wpf/files/3425550/Build.zip)