[C++11] Update Clang for the change to LLVM's Use-Def chain iterators in

[C++11] Update Clang for the change to LLVM's Use-Def chain iterators in
r203364: what was use_iterator is now user_iterator, and there is
a use_iterator for directly iterating over the uses.

This also switches to use the range-based APIs where appropriate.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203365 91177308-0d34-0410-b5e6-96231b3b80d8