Enhance the libc++ list data formatter so that it does not start looking for...

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

llvm-svn: 219447