Portable Python script across Python version

Make scripts more future-proof by importing most __future__ stuff.

Differential Revision: https://reviews.llvm.org/D55208

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@349504 91177308-0d34-0410-b5e6-96231b3b80d8
23 files changed