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
Patch by Amy Huang!
Differential Revision: https://reviews.llvm.org/D59440