[clang][Analysis] CallGraph: store the actual call `Expr*` in the CallGraphNode::CallRecord

Summary:
Storing not just the callee, but the actual call may be interesting for some use-cases.
In particular, D72362 would like that to better pretty-print the cycles in call graph.

Reviewers: NoQ, erichkeane

Reviewed By: NoQ

Subscribers: martong, Charusso, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74081