WPF Application Always Rebuilds In Visual Studio

### Background
In attempting to reproduce the issue for #4217 I attempted to create a fresh WPF Project to see if I could reproduce it; in doing this I encountered a similar (but I do not believe identical) issue wherein the MainWindow.g.cs was always regenerated.

### Steps to reproduce
Please find an attached zip file that contains the project that demonstrates this behavior: [WpfAppAlwaysRebuilds.zip](https://github.com/Microsoft/msbuild/files/2956954/WpfAppAlwaysRebuilds.zip)

This project was created by:

1. Creating a new WPF Application within Visual Studio
2. Modifying the CSPROJ To contain the work around for #1648 as follows
```xml



```

To reproduce the issue on the command line build the project

```
msbuild WpfAppAlwaysRebuilds.sln /t:Build
```

Then launch the Solution in Visual Studio and build you will see this in diganostic verbosity:

```
Project 'WpfAppAlwaysRebuilds' is not up to date. Input file 's:\temp\markupbug\wpfappalwaysrebuilds\wpfappalwaysrebuilds\obj\debug\mainwindow.g.cs' is modified after output file ''.
------ Build started: Project: WpfAppAlwaysRebuilds, Configuration: Debug Any CPU ------
```

Note that in order to repeat it a second time you MUST delete the contents of the /obj/ folder to start "clean" again. Once it has built in Visual Studio once MSBuild will happily report that it is up to date.

### Expected behavior
The expected behavior is that this project will report as "up-to-date"

### Actual behavior
The project rebuilds because it determines that it is not "up-to-date"

### Environment data
`msbuild /version` output:
```
Microsoft (R) Build Engine version 15.9.21+g9802d43bc3 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

15.9.21.664
```

OS info:
Windows Server 2012 R2

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):
Visual Studio 15.9.8