Don't raise an exception if we can't find the src assembly on a shadow copy appdomain

This is all related to https://bugzilla.xamarin.com/show_bug.cgi?id=17066

But to summarise.

Currently, attempting to call Type.GetType("A.B.T, NonExistantAssembly") will normally return null.  BUT if you attempt to do this within an appdomain with ShadowCopyFile="true" you will instead get a RuntimeException.

This patch calls GetFileAttributes in an attempt to ensure that the src file is readable by caller and if not just return the existing filename.