Add data formatter for libc++'s forward_list

Summary:
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

llvm-svn: 316992