Add proto serialization/deserialization testing to the HLO parser tests.

Many of the HLO parser tests verify that an text form of an HLO module preserves all information when running through ToString then parsing. It makes sense to also use these tests to exercise proto serialization/deserialization. This is done by adding additional instantiations of the parameterized parsing tests. This caught several bugs which are fixed in this CL:

(1) Domain instructions were not being serialized properly.
(2) Host send/recv instructions did not preserve the is_host_transfer bit.
(3) Sparse literals could not be serialized or deserialized.

PiperOrigin-RevId: 215445200