[opt-viewer] Python 3 support in opt-diff.py

Summary:
The `file()` builtin is not available in Python 3; use `open()` instead.
https://docs.python.org/3.0/whatsnew/3.0.html#builtins

Reviewers: anemet, davidxl, davide

Reviewed By: davide

Subscribers: davide, fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D34670

llvm-svn: 306423