Add support for the --xla_dump_module_metadata flag to GPU backend.
Add support for the --xla_dump_module_metadata flag to GPU backend.

So far, this flag was only used for the TPU backend, but it can be useful for
the GPU backend, too. The module metadata dump contains timing information for
every HLO pass together with the pass name, the pipeline name and whether the
module was changed by the pass.
