Save function argument names as locs through MHLO<->HLO conversion.
Save function argument names as locs through MHLO<->HLO conversion.

JAX attaches the name of the function argument to function arguments in the MLIR, see the `loc("x")`:
```
@main(%arg0: tensor<8x8xf32> {mhlo.layout_mode = "default"} loc("x"))
```
But these aren't preserved during MHLO->HLO->MHLO round-tripping. This adds support for it.
