[clang][dataflow] Extend transfer functions for other `CFGElement`s

Previously, the transfer function `void transfer(const Stmt *, ...)` overriden by users is restricted to apply only on `CFGStmt`s and its contained `Stmt`.

By using a transfer function (`void transfer(const CFGElement *, ...)`) that takes a `CFGElement` as input, this patch extends user-defined analysis to all kinds of `CFGElement`. For example, users can now handle `CFGInitializer`s where `CXXCtorInitializer` AST nodes are contained.

Reviewed By: gribozavr2, sgatev

Differential Revision: https://reviews.llvm.org/D131614