Add Python 3 support to clang.cindex

Summary:
Introduce an interop string to convert from unicode to c-strings where needed.
Add missing conversions from _CXString to strings in function registrations.
Explicitly evaluate lists where Python 3's lazy iterators would not otherwise do so.

This is an improvement upon the reverted change proposed in https://reviews.llvm.org/D26082

Reviewers: compnerd, skalinichev, modocache, MathieuDuponchelle

Reviewed By: compnerd

Subscribers: cfe-commits

Tags: #clang-c

Differential Revision: https://reviews.llvm.org/D31568

llvm-svn: 300829