[CallGraph] Add support for callback call sites

Summary:
This patch changes call graph analysis to recognize callback call sites
and add an artificial 'reference' call record from the broker function
caller to the callback function in the call graph. A presence of such
reference enforces bottom-up traversal order for callback functions in
CG SCC pass manager because callback function logically becomes a callee
of the broker function caller.

Reviewers: jdoerfert, hfinkel, sstefan1, baziotis

Reviewed By: jdoerfert

Subscribers: hiraditya, kuter, sstefan1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82572