[Regression] Unqualified expansion isn't expanding in some cases

Build: 26107.1 D15PreRel

This appears to be a regression, and is succeeding on certain builds of VS but failing on others.

I attempted to break down the repro but it does not repro outside of our environment

1. git clone https://github.com/dotnet/roslyn-project-system.git
2. cd roslyn-project-system
3. git checkout 7ad11b8
4. build

Expected: Build to succeed
Actual:

```
E:\project-system\build\Targets\VSL.Imports.targets(559,5): error MSB3030: Could not copy the file "\**\" because it was not found. [E:\project-system\src\Dependencies\Immutable\Immutable.csproj]
E:\project-system\build\Targets\VSL.Imports.targets(559,5): error MSB3030: Could not copy the file "\**\" because it was not found. [E:\project-system\src\Dependencies\CoreFX\CoreFX.csproj]
E:\project-system\build\Targets\VSL.Imports.targets(559,5): error MSB3030: Could not copy the file "\**\" because it was not found. [E:\project-system\src\Dependencies\xUnit.net\xUnit.net.csproj]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\bin\Microsoft.Common.CurrentVersion.targets(4604,5): error MSB3541: Files has invalid value "E:\project-system\bin\Debug\\Rules\**\". Illegal characters in path. [E:\project-system\src\Dependencies\CoreFX\CoreFX.csproj]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\bin\Microsoft.Common.CurrentVersion.targets(4604,5): error MSB3541: Files has invalid value "E:\project-system\bin\Debug\\Rules\**\". Illegal characters in path. [E:\project-system\src\Dependencies\Immutable\Immutable.csproj]
```

The target that is failing is this:

``` XML

<_XamlPropertyRuleToCopy Include="@(XamlPropertyRule);@(XamlPropertyRuleNoCodeBehind);@(XamlPropertyProjectItemsSchema)" />

<_XamlPropertyRuleToCopy Include="%(RootDir)%(Directory)\**\%(Filename)%(Extension)" />

```

In particular:

``` XML
<_XamlPropertyRuleToCopy Include="%(RootDir)%(Directory)\**\%(Filename)%(Extension)" />
```

Is expanding to:

```
\**\
```

Which appears to be invalid item spec (I think?), resulting in the trailing copy to fail.