Add Python 3 support to clang.cindex
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
Differential Revision: https://reviews.llvm.org/D31568