Repro steps:

1. cd Open\src\Compilers\Core\Portable
2. MSBuild /m /p:Configuration=Release
3. Repeat step 2.
   Result:
   Build fails with:
   EXEC : error : The module D:\Roslyn\Open\src\Compilers\Core\Portable\obj\Release\Microsoft.CodeAnalysis.dll, already contains an IBC resource. [D:\Roslyn\Open\src\Compilers\Core\Portable\CodeAnalysis.csproj]

The problem is that the FakeSign step has modified the DLL after IBCMerge, thwarting the .timestamp file that each step writes out.

We need to come up with a technique so that multiple 'rewriters' can modify the binary and keep incremental builds working.