Correct value_type for some TrackingAllocators.

The standard says that the value_type for the allocator of a
map/multimap must be std::pair<const Key, T>. These instances were not
properly carrying the const, and the updated version of libc++ has a
static_assert to make sure that map::allocator_type and
allocator::value_type really do match (as mismatches will tickle
undefined behavior in libc++).

Change-Id: I7a3a49fccea4ecc4579929d9ddd7872c62b60f5f