Fix some failing tests for the standard containers. The tests were failing in 32-bit mode because they assumed that std::size_type and make_unsigned<ptrdiff_t>::type were always the same type. No change to libc++, just the tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@212538 91177308-0d34-0410-b5e6-96231b3b80d8