[analyzer][NFC] Introduce CXXDeallocatorCall, deploy it in MallocChecker

One of the pain points in simplifying MallocCheckers interface by gradually
changing to CallEvent is that a variety of C++ allocation and deallocation
functionalities are modeled through preStmt<...> where CallEvent is unavailable,
and a single one of these callbacks can prevent a mass parameter change.

This patch introduces a new CallEvent, CXXDeallocatorCall, which happens after
preStmt<CXXDeleteExpr>, and can completely replace that callback as
demonstrated.

Differential Revision: https://reviews.llvm.org/D75430