Incremental compilation should not require an output file map

Several FIXME in the Swift repository mention that compilation with -incremental should not require the -output-file-map option to be specified:

• docs/DriverInternals.rst#L61-L62
• lib/Driver/Driver.cpp#L639-L640

Instead, the notes indicate that another way to specify a build record and where to put intermediates should be used. What should this other way be? What are the downsides of using an output file map?