Use llvm::next() instead of incrementing begin iterators of std::vector.

Iterator of std::vector may be implemented as a raw pointer. In
this case begin iterators are rvalues and cannot be incremented.
For example, this is the case with STDCXX implementation of vector.

Patch by Konstantin Tokarev <annulen@yandex.ru>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189911 91177308-0d34-0410-b5e6-96231b3b80d8