[clang-format] update trailing newline treatment in clang-format.py

Summary:
The current clang-format.py does not handle trailing newlines at the end of a file correctly.
Trailing empty lines get removed except one.
As far as I understand this is because clang-format gets fed from stdin and writes to stdout when called from clang-format.py.
In a "normal" file (with no trailing empty lines) the string that gets passed to clang-format does not contain a trailing '\n' after the '\n'.join from python.
The clang-format binary does not add a trailing newline to input from stdin, but (if there are multiple trailing '\n', all except one get removed).

When reading back this means that we see in python from a "normal" file a string with no trailing '\n'. From a file with (potentially multiple) empty line(s) at the end, we get a string with one trailing '\n' back in python. In the former case all is fine, in the latter case split('\n') makes one empty line at the end of the file out of the clang-format output. Desired would be instead that the **file** ends with a newline, but not with an empty line.

For the case that a user specifies a range to format (and wants to keep trailing empty lines) I did **not** try to fix this by simply removing all trailing newlines from the clang-format output. Instead, I add a '\n' to the unformatted file content (i.e. newline-terminate what is passed to clang-format) and then strip off the last newline from the output (which itself is now for sure the newline termination of the clang-format output).

(Should this get approved, I'll need someone to help me land this.)

Reviewers: klimek, MyDeveloperDay

Reviewed By: MyDeveloperDay

Patch By: pseyfert

Subscribers: cfe-commits, llvm-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D70864
update trailing newline treatment in clang-format.py