interesting feature to add to clang

[Reed Kotler]

http://homepage.ntlworld.com/jonathan.deboynepollard/Proposals/metaware-iterator-driven-for.html

I wish I had the spare cycles right now to add this feature.

This form of iterators really simplifies many complex algorithms in 
compilers; especially many graph algorithms.

In general it works much better than STL form iterators in terms of 
algorithm clarity and simplification, IMHO.