[libomptarget] Fix compilation with libc++

iterator is only guaranteed to be default-constructible, without any argument.

Differential Revision: https://reviews.llvm.org/D29171

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@293277 91177308-0d34-0410-b5e6-96231b3b80d8