Add data formatter for libc++'s forward_list
This adds a data formatter for the implementation of forward_list in
libc++. I've refactored the existing std::list data formatter a bit to
enable more sharing of code (mainly the loop detection stuff).
Reviewers: jingham, EricWF
Subscribers: srhines, eugene, lldb-commits
Differential Revision: https://reviews.llvm.org/D35556