[XLA] Add literal C64/C128 support to HLO parser.

Now the parser can handle complex numbers.

In addition, this patch fixes a few bugs I discovered:

 - We weren't checking for out-of-range values in sparse literals.
 - We were saying that bf16 values > F16_MAX were invalid, but in fact bf16 has
   a higher dynamic range than this.
 - We were accepting invalid sparse indices.  Strangley this doesn't crash or
   anything.

PiperOrigin-RevId: 231492104