As discussed on IRC, for function importing we don't need to link in
the type, enums and many other metadata hanging off the DICompileUnit
for the source module. With this patch, we will only map in the needed
DISubprograms and the DIFile (which is required) and a few simple
Do we also need to link in the DIImportedEntity though? We are marking any
DISubprograms reached from them as needed (findNeededSubprograms()).