[libclang] Add support for querying cursor availability

Summary:
This patch allows checking the availability of cursors through libclang and clang.cindex (Python).
This e.g. allows to check whether a C++ member function has been marked as deleted.

Reviewers: arphaman, jbcoe

Reviewed By: jbcoe

Subscribers: cfe-commits

Tags: #clang

Patch by jklaehn (Johann Klähn)

Differential Revision: https://reviews.llvm.org/D36973

llvm-svn: 315959