Handle two corner cases in creduce-clang-crash.py

Summary:
First, call os.path.normpath on the filename argument. I passed in
./foo-asdf.cpp, and this meant that the script failed to find the
filename, and bad things happened.

Second, call os.path.abspath on binaries. CReduce runs the
interestingness test in a temp dir, so relative paths will not work.

Reviewers: akhuang

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71098