Add handleLoadedFile hook to the context.

This is called from the resolver on each file we decide we actually want to use.

Future commits will make use of this to extract useful information from the files and do
error checking against the context.  For example, ensure that files are the same arch as
each other.

Reviewed by Lang Hames.

Differential Revision: http://reviews.llvm.org/D16093

llvm-svn: 257814