[formatters] Add a libstdcpp formatter for forward_list and refactor list formatter

This diff adds a data formatter for libstdcpp's forward_list. Besides, it refactors the existing code by extracting the common functionality between libstdcpp forward_list and list formatters into the AbstractListSynthProvider class.

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D113362