[llvm][dfsan][NFC] Factor out fcn initialization

Summary:
Moving these function initializations into separate functions makes it easier
to read the runOnModule function. There is also precedent in the sanitizer code:
asan has a function ModuleAddressSanitizer::initializeCallbacks(Module &M). I
thought it made sense to break the initializations into two sets. One for the
compiler runtime functions and one for the event callbacks.

Tested with: check-all

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D79307