`rehydrateIface` only exists in a Note

The Rehydrating Modules note says:

Solution: rehydration. Before compiling R.hs, rehydrate all the ModIfaces below it that depend on R.hs-boot. To rehydrate a ModIface, call rehydrateIface to convert it to a ModDetails. It's just a de-serialisation step, no type inference, just lookups.

This note was added in 91c0a657 and as of 89cb1315, there is still no rehydrateIface function in the entire GHC codebase. Note that this is not a case of the note going out of date: there's no commit since 91c0a657 that contained this function.