Enhance the libc++ list data formatter so that it does not start looking for loops until asked to actually fetch children. Also, if you're going to read child X, only look for a loop in the first X nodes. Loops further down the road won't really matter. This should speed things up for large lists and fix rdar://18583790

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@219447 91177308-0d34-0410-b5e6-96231b3b80d8