[XLA:MSA] Move `BufferIntervalComparator` and its implementations `MemoryBoundednessBufferIntervalComparator` and `DefaultCrossProgramPrefetchBufferIntervalComparator` to  `buffer_interval_comparator.h/cc` files under `memory_space_assignment` directory.
[XLA:MSA] Move `BufferIntervalComparator` and its implementations `MemoryBoundednessBufferIntervalComparator` and `DefaultCrossProgramPrefetchBufferIntervalComparator` to  `buffer_interval_comparator.h/cc` files under `memory_space_assignment` directory.
