Driver: Name crashdump scripts after the first temp file

In practice there's only ever one temporary output file when
generating a crashdump, but even if there were many iterating over
each and creating a duplicate run script for each one wouldn't make
very much sense.

This updates the behaviour to only generate the script once, based on
the first filename.

This should make it more reasonable to generate extra output files to
include in the crashdump going forward, so I've also added a FIXME to
look into doing just that with the extra module crashdump files.

llvm-svn: 220238