Various fixes and additions to creduce-clang-crash.py

Some more additions to the script - mainly reducing the clang args after
the creduce run by removing them one by one and seeing if the crash
reproduces. Other things:

- remove the --crash flag when "fatal error" occurs
- fixed to read stack trace functions from the top
- run creduce on a copy of the original file

Patch by Amy Huang!

Differential Revision: https://reviews.llvm.org/D59725

llvm-svn: 357290