How to write a custom intermodular pass in LLVM?

Question

I've written a standard Analysis pass in LLVM, by extending the FunctionPass class. Everything seems to make sense.

Now what I'd like to do is write a couple of intermodular passes, that is, passes that allows me to analyze more than one module at a time. The purpose of one such pass is to construct a call graph of the entire application. The purpose of the other such pass is that I have an idea for an optimization involving function calls and their parameters.

I know about interprocedural passes in LLVM, via extending the ModulePass class, but that only allows analysis within a single module.

I know about link time optimization (LTO) in LLVM, but (a) I'm not quite clear if this is what I want and (b) I've found no examples or documentation on how to actually write an LTO pass.

How can I write an intermodular pass, i.e., a pass that has access to all the modules in an application, in LLVM?