[CallSite removal][IPO] Change implementation of AbstractCallSite to store a...

[CallSite removal][IPO] Change implementation of AbstractCallSite to store a CallBase* instead of CallSite. NFCI.

CallSite will likely be removed soon, but AbstractCallSite serves a different purpose and won't be going away.

This patch switches it to internally store a CallBase* instead of a
CallSite. The only interface changes are the removal of the getCallSite
method and getCallBackUses now takes a CallBase&. These methods had only
a few callers that were easy enough to update without needing a
compatibility shim.

In the future once the other CallSites are gone, the CallSite.h
header should be renamed to AbstractCallSite.h

Differential Revision: https://reviews.llvm.org/D78322