[libcxx] [test] Fix an MSVC x64 compiler error due to mismatched iterator types.

[STL_MSFT]

Summary

Fix an MSVC x64 compiler error due to mismatched iterator types.

This was attempting to store a list<int, test_allocator<int>>::iterator in a list<int>::iterator. That isn't guaranteed by the Standard, and MSVC x64 rejects it (even though our list iterators are SCARY) because test_allocator has 32-bit size_type/difference_type, and our iterators take that into account even as they erase the allocator type itself.

[EricWF]

r277574.

test/std/containers/sequences/list/list.cons/initializer_list_alloc.pass.cpp

 
 int main()
 {
 #ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS
     {
     std::list<int, test_allocator<int>> d({3, 4, 5, 6}, test_allocator<int>(3));
     assert(d.get_allocator() == test_allocator<int>(3));
     assert(d.size() == 4);
-    std::list<int>::iterator i = d.begin();
+    std::list<int, test_allocator<int>>::iterator i = d.begin();
     assert(*i++ == 3);
     assert(*i++ == 4);
     assert(*i++ == 5);
     assert(*i++ == 6);
     }
 #if TEST_STD_VER >= 11
     {
     std::list<int, min_allocator<int>> d({3, 4, 5, 6}, min_allocator<int>());