[clang-rename] make clang-rename.py vim integration python3 compatible

Summary:
This patch makes the clang-rename.py script useable for vim with only python3
support. It uses the print-function and adjust the doc slightly to mention
the correct python3 command for the letter mapping in vim.

Reviewers: arphaman, klimek, aaron.ballman, hokein

Reviewed By: hokein

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50307

llvm-svn: 338996