[AbstractCallSite] Look though constant cast expression when checking for callee use

Summary: That makes AbstractCallSite::isCallee(const Use *) behavior consistent with AbstractCallSite constructor.

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79188