Binding functions (whether they be translated CFGs or overrides) in the
LLVM memory model is a bit more complex than in other frontends. We had
organically grown a number of helpers that take care of various parts of
this process, without actually documenting what needs to happen for
everything to work as expected. This commit collects those helpers into
a single module, with a top-level comment that outlines the whole
process.

Along the way, it removes some superfluous helpers (`bindLLVMFunPtr`)
and introduces some new ones (`bindLLVMCFG`).