I found that an imported instance was getting printed with <no
location info>.  Fixing this pushed me into a bit more refactoring
than I intended, but it's all small aesthetic stuff, nothing
fundamental.  Caused some error message to change as a result.

I removed pprDefnLoc from the GHC API because it doesn't seem to be
used.  Name.pprNamedefnLoc and pprDefinedAt are probably more useful
anyway.