The first API is clang_visitCXXBaseClasses: this allows visiting the
base classes without going through the generic child visitor (which is
awkward, and doesn't work for template instantiations).
The second API is clang_getOffsetOfBase; this allows computing the
offset of a base in the class layout, the same way
clang_Cursor_getOffsetOfField computes the offset of a field.
Also, add a Python binding for the existing function
clang_isVirtualBase.
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