bindings: add new C++ function attribute accessors

Add methods to index Cursor to see if a cxx method is pure_virtual,
virtual or const methods.

Patch by Jonathan B Coe!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@250008 91177308-0d34-0410-b5e6-96231b3b80d8