Cleaning of project in VS does not clean completely

[Marv51]

Describe the bug I was having some XAML issues, when I cleaned the solution, the problem persisted. When I ran git clean -xfd the problem disappeared. So independent of the issue I was looking at, clearly cleaning the solution does not work properly.

I have verified that even after running clean twice the obj folder is far from empty:

> tree /F
obj
│   BrandMatrix.Modern2.csproj.nuget.dgspec.json
│   BrandMatrix.Modern2.csproj.nuget.g.props
│   BrandMatrix.Modern2.csproj.nuget.g.targets
│   project.assets.json
│
└───x64
    └───Debug
        └───net5.0-windows10.0.19041.0
            │   .NETCoreApp,Version=v5.0.AssemblyAttributes.cs
            │   apphost.exe
            │   BrandMatrix.Modern2.assets.cache
            │   BrandMatrix.Modern2.csproj.FileListAbsolute.txt
            │
            ├───DocumentationGeneration
            ├───embed
            │       embed.resfiles
            │       embed.resfiles.intermediate
            │
            ├───Generated Files
            │       cswinrt.rsp
            │       WinRTEventHelpers.cs
            │
            ├───intermediatexaml
            │       BrandMatrix.Modern2.dll
            │       BrandMatrix.Modern2.pdb
            │
            ├───Licenses
            ├───ref
            └───View
                    FlyInStyles.g.cs
                    FlyInStyles.g.i.cs

Expected behavior Manually cleaning the solution should delete all build outputs.

Version Info Visual Studio 16.10.3

NuGet package version: WinUI 3 - Windows App SDK 0.8: 0.8.0

Windows app type:	UWP	Win32
	Yes

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
21H1 (19043)	Yes
October 2020 Update (19042)
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

Additional context Binary clean build log (.binlog): _Debug_x64_Build_2021-07-08T14_23_14.1019256+02_00.zip

[StephenLPeters]

I've seen this issue with many versions of VS while working in Winui2 as well.

[evelynwu-msft]

It does seem like we could be doing a better job of recording intermediate files that get generated during the build!

[Marv51]

Keep alive (re #8638)