update string comparison in clang-format.py

Summary: Python 3.8 introduces a SyntaxWarning about string comparisons with 'is'. This commit updates the string comparison in clang-format.py that is done with 'is not' to '!='. This should not break compatibility with older python versions (tested 3.4.9, 2.7.17, 2.7.5, 3.8.0).

Reviewers: MyDeveloperDay, klimek, llvm-commits, cfe-commits

Reviewed By: MyDeveloperDay, klimek

Patch By: pseyfert

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D70664