Add class abstractness checking

[RamblingMadMan]

A pretty simple feature to add that is already supported by libclang with clang_CXXRecord_isAbstract.

[saraedum]

@RamblingMadMan it seems that your editor is using tabs instead of spaces. You might want to rerender these files with spaces to match the files that you modified.

[saraedum]

Closing this one since nobody worked on it for years. Feel free to reopen.