C#/WinRT doesn't properly incremental build

[stevenbrix]

If I change an .idl file in my C++ project, when I do an incremental build, the C#/WinRT projection isn't updated. This is the opposite problem of #519.

Repro project: CS-Desktop2.zip

Repro steps:

1. Do a full build
2. In the RuntimeComponent1 project, change Class.idl to take a constructor which accepts an Int32:
   Class(Int32 prop);
3. Do an incremental build

Expected results
Build fails, since call sites in C# code is using the default constructor, which no longer exists

Actual results
Build succeeds

[Scottj1s]

Problem doesn't occur from command line (msbuild), so we likely need to add tlog entries for cswinrt.exe executions