[Docs] Update CodingStandards to recommend range-based for loops

The CodingStandards section on avoiding the re-evaluation of end() hasn't been
updated since range-based for loops were adopted in the LLVM codebase. This
patch adds a very brief section that documents how range-based for loops
should be used wherever possible. It also moves example code in
CodingStandards to use range-based for loops and auto when appropriate.

Differential Revision: https://reviews.llvm.org/D37264

llvm-svn: 312236