Clean target should delete files under obj directory

[AArnott]

git clone https://github.com/microsoft/vs-validation.git
cd vs-validation
git checkout acdf1788a
cd src\Microsoft.VisualStudio.Validation
msbuild /t:restore
msbuild
msbuild /t:clean

dir obj\Debug\net45

Expected

An empty directory

Actual

These files exist:

CoreCompileInputs.cache
Microsoft.VisualStudio.Validation.AssemblyInfo.cs
TemporaryGeneratedFile_036C0B5B-1481-4323-8D20-8F5ADCB23D92.cs
TemporaryGeneratedFile_5937a670-0e60-4077-877b-f7221da3dda1.cs
TemporaryGeneratedFile_E7A71F73-0F8D-4B9B-B56E-8E70B10BC5D3.cs

Of particular importance is the generated AssemblyInfo.cs file that remains, as that's a generated file and incremental build doesn't update it properly, so a clean build should definitely remove it.

[rainersigwald]

The first one is from MSBuild's common targets: dotnet/msbuild#1577.

[nguerrera]

@AArnott I'm not able to reproduce this either on d15prerel latest. The only file that remains is CoreCompileInputs.cache.

[nguerrera]

This may have been what fixed it: #276

Does it still repro for you on latest bits?

[AArnott]

It no longer repros for me with dotnet 1.0.0-rc4-004600.
Thanks.