Reverting llvm::distance changes to use std::distance with iterators instead, per post-commit review feedback.

Replacing llvm::copy changes with SmallVector range-based construction which is a considerably cleaner approach.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203461 91177308-0d34-0410-b5e6-96231b3b80d8