[XLA] Initial sparse layout support

Adds SparseIndexArray and support methods to Literal. SparseIndexArray manages
the array of sparse indices and is exposed by sparse Literals.

Also adds HloSupportChecker classes for CPU and GPU.  This will be run as the
first HloPass during compilation, and verifies that the graph is supported by
the backend.  Currently only verifies shapes, and that the layout is not sparse
since no backend supports sparse layouts yet.

PiperOrigin-RevId: 181244401