Migrate utils/ Python 2 scripts to Python 3

Most if not all Python scripts in the clang/ repo have been ported and are waiting for reviews.
I've split them in independant patches to ease the review works:

https://reviews.llvm.org/D55259
https://reviews.llvm.org/D55258
https://reviews.llvm.org/D55213
https://reviews.llvm.org/D55208
https://reviews.llvm.org/D55205
https://reviews.llvm.org/D55204
https://reviews.llvm.org/D55202
https://reviews.llvm.org/D55200
https://reviews.llvm.org/D55199
https://reviews.llvm.org/D55197
https://reviews.llvm.org/D55196
https://reviews.llvm.org/D55193
https://reviews.llvm.org/D55194

Have fun with these!

Hi folks,

It is my great pleasure to announce that most-if-not-all Python scripts from llvm,
clang and test-suite have been updated to work (hopefully) the same way
on Python2 and Python3.

As a consequence, all shebangs now points to python, and not python2 or
python3, and the documentation should now refer to python, whith no
specified version.

So what's next? Maybe some buildbot could be updated to specifically
build with a given Python version, if it's not already the case.

I'll also added an herald rule to review commited python scripts, to
ensure the compatible property remains true over time.

Oh, and I *really* would like to thanks Michael Platings for the tremendous
reviewing job he's been doing on this patch set o/

Awesome, that’s great news! Thanks!