[MLton] MLB export filters and -show-def-use

[Vesa Karvonen]

Another, well, inconvenience I noticed while working on the def-use mode is
that (non-renaming) MLB export filters partition the def-use information in
an inconvenient way. Consider the following two pieces of def-use info:

  signature MLTON lib/mlton/sml/basis/libs/basis-extra/top-level/basis-sigs.sml 85.11
      lib/mlton/sml/basis/mlton.mlb 17.17

  signature MLTON lib/mlton/sml/basis/mlton.mlb 17.17

Notice that the first one says that MLTON is used in mlton.mlb while the
second says that MLTON is defined at the same point.  This is correct, but
this is also inconvenient in that otherwise there would be no need to handle
the possibility of overlapping definitions and uses.  (Currently, in def-use
mode, you would not be able to jump to the definition in basis-sigs.sml from
mlton.mlb.  This could, of course, be fixed in def-use mode.)  Perhaps a
better way to handle non-renaming export filters would be to treat them as
plain uses (and not as definitions).

-Vesa Karvonen