Only read output_file_map if it's going to be written
Only read output_file_map if it's going to be written

The output_file_map is only written to disk in a transformed state if an incremental build is happening. This change causes the output_file_map to only be read and processed in that case as well, leading to less work in the WMO case.

bazelbuild/rules_swift,bazelbuild/rules_swift