[libcxx] Fix incorrect iterator type in vector container test

The iterator types for different specializations of containers with the
same element type but different allocators are not required to be
convertible. This patch makes the test to take the iterator type from
the same container specialization as the created container.

Reviewed as https://reviews.llvm.org/D54806.
Thanks to Andrey Maksimov for the patch.

llvm-svn: 347423