Some updates/fixes to the creduce script.

This was motivated by changes to llvm's `not --crash` disabling symbolization
but I ended up removing `not` from the script entirely because it
returns differently depending on  whether clang "crashes" or exits for some
other reason. The script had to choose between calling `not` and `not --crash`
and sometimes it was wrong.

The script also now disables symbolization when we don't read the stack
trace because symbolizing is kind of slow.

Differential Revision: https://reviews.llvm.org/D91372