[DataFormatters] Adjusting libc++ std::list formatter to act better with pointers...

[DataFormatters] Adjusting libc++ std::list formatter to act better with pointers and references and adding a test to cover a previous related fix

Summary:
This previous fix https://github.com/llvm-mirror/lldb/commit/5469bda296c183d1b6bf74597c88c9ed667b3145 did not have a test since we did not have a reproducer.

This is related to how formatters deal with pointers and references. The added tests both the new behavior and covers the previous bug fix as well.

Differential Revision: https://reviews.llvm.org/D60588

llvm-svn: 359118