[MLton] Selectors in -show-def-use output

[Vesa Karvonen]

Quoting Matthew Fluet <fluet at tti-c.org>:
[...]
> So, it is probably not the right choice to expose the dummy variable in 
> the def-use information.  Somewhere in elaborate-core.fun is probably 
> the right place to fix this.

Browsing the code, I realized that bindings whose definition has a bogus
region are not reported.  So, I changed the selector function to use a
bogus region for the generated variable:

  http://mlton.org/pipermail/mlton-commit/2007-February/001159.html

The enclosing pattern, however, is still given a non-bogus region and I
checked that the compiler seems to report unresolved record patterns
correctly.  BTW, browsing the code (namely uses of Symbol.fromString),
I noticed that there may be other places where it might be appropriate
to use a bogus region.

-Vesa Karvonen