creduce-clang-crash.py: preprocess file + reduce commandline

This CL causes our creduce-clang-crash.py util to:

- try to preprocess the file before reducing
- try to remove some command line arguments
- now require a llvm bin directory, since the generated crash script
  doesn't have an absolute path for clang

It also marks it as executable, since I forgot to do that in the last
commit. :)

Patch by Amy Huang!

Differential Revision: https://reviews.llvm.org/D59440

llvm-svn: 356636