Driver: Keep -isysroot flags in crash scripts if we're dumping a VFS

For crashes with a VFS (ie, with modules), the -isysroot flag is often
necessary to reproduce the crash. This is especially true if some
modules need to be rebuilt, since without the sysroot they'll try to
read headers that are outside of the VFS.

I find it likely that we should keep some of the other -i flags in
this case as well, but I haven't seen that come up in practice yet so
it seems better to be conservative.

llvm-svn: 231997