Build/Rebuild doesn't generate XML documentation if DLL is up to date

[DustinCampbell]

Refiling for @MV10 on roslyn-project-system

Can't recall whether VS has always worked this way, I've probably never tried this particular scenario, but it seems like a bug.

Version Used:
VS 2015 Community edition with .NET Core 1.0 release and Tools Preview 2

Steps to Reproduce:

1. Create a .NET Core class project
2. Build
3. Open project properties
4. Check the "XML documentation file" option
5. Rebuild

Expected Behavior:
XML documentation file will be generated in the framework bin folders.

Actual Behavior:
No docs are generated. I have to go into the bin folders and delete the DLL (or change the source) to force a rebuild before the XML file is created.

[davkean]

@MV10 This is a bug/limitation in what we call the "fast up-to-check" which for performance reasons bypasses msbuild to do a quick up-to-date check. We're tracking for the rewrite under this feature: #62.