[libclang] Add support for querying cursor availability
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
Patch by jklaehn (Johann Klähn)
Differential Revision: https://reviews.llvm.org/D36973