XamlBuildTask.dll is not in $(MSBuildToolsPath64)

### Steps to reproduce

https://github.com/KirillOsenkov/SourceBrowser/files/2611685/WorkflowConsoleApplication1.zip

### Problem:
https://github.com/Microsoft/msbuild/blob/701ed49b20b860c17bcfd62d4e7845647a590760/src/Tasks/Microsoft.Xaml.targets#L16
It says that XamlBuildTask.dll should be in `$(MSBuildToolsPath64)`, which on my machine is `C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\amd64`, but XamlBuildTask.dll isn't there. It's only found in the GAC and Reference Assemblies:

```
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.2\XamlBuildTask.dll
...
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\XamlBuildTask.dll
C:\Windows\Microsoft.NET\Framework64\v4.0.30319\XamlBuildTask.dll
C:\Windows\Microsoft.NET\Framework\v4.0.30319\XamlBuildTask.dll
C:\Windows\Microsoft.NET\assembly\GAC_MSIL\XamlBuildTask\v4.0_4.0.0.0__31bf3856ad364e35\XamlBuildTask.dll
```

### Actual behavior
```
"C:\Temp\WorkflowConsoleApplication1\WorkflowConsoleApplication1\ActivityLibrar
y1\ActivityLibrary1.csproj" (default target) (2) ->
(XamlMarkupCompilePass1 target) ->
C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Microsoft.Xaml.targets(193,5):
error MSB4062: The "Microsoft.Build.Tasks.Xaml.PartialClassGenerationTask" task
could not be loaded from the assembly C:\Program Files (x86)\Microsoft Visual
Studio\2017\Enterprise\MSBuild\15.0\bin\amd64\XamlBuildTask.dll. Could not load
file or assembly 'file:///C:\Program Files (x86)\Microsoft Visual Studio\2017\
Enterprise\MSBuild\15.0\bin\amd64\XamlBuildTask.dll' or one of its dependencies
. The system cannot find the file specified. Confirm that the decla
ration is correct, that the assembly and all its dependencies are available, an
d that the task contains a public class that implements Microsoft.Build.Framewo
rk.ITask. [C:\Temp\WorkflowConsoleApplication1\WorkflowConsoleApplication1\Acti
vityLibrary1\ActivityLibrary1.csproj]
```

### Environment data
`msbuild /version` output:

```
Microsoft (R) Build Engine version 15.9.20+g88f5fadfbe for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.
15.9.20.62856
```