adapt usage of CXXMethodDecl::isPure for LLVM>=18
since LLVM18 isPure was renamed to isPureVirtual
see: https://github.com/llvm/llvm-project/commit/e90e43fb9cd1d305f7196cd526aa503374e0f616
since LLVM18 isPure was renamed to isPureVirtual
see: https://github.com/llvm/llvm-project/commit/e90e43fb9cd1d305f7196cd526aa503374e0f616