Expose clang_getOverriddenCursors to python clang module

[Andrew Nelson via cfe-commits]

This patch exposes an interface to libclang's clang_getOverriddenCursors
and clang_disposeOverriddenCursors functions to the python api.

The patch is adapted from this stack overflow response but cleaned up to
fit within the existing cindex.py codebase
https://stackoverflow.com/questions/35962473/libclang-python-binding-function-getoverridden

Let me know if there's anything I need to address.  I can add unit tests if
desired but that would double the length of this patch.

Patch attached
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