Fix Python 3 language issues and add an explicit check for Python version == 2.

Summary:
Python bindings cannot support Python 3 without work being done to fix Unicode c-string conversion.

This was attempted in https://reviews.llvm.org/D26082. That patch was reverted due to memory access issues on Linux.

This revision fixes enough language compatibility issues for the clang module to be loaded and raise an error if the Python version is not 2.

Reviewers: mgorny, MathieuDuponchelle, rengolin, compnerd

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D28682

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@292247 91177308-0d34-0410-b5e6-96231b3b80d8