Support for LLVM 8.0.0

We are currently working on support for LLVM 8. Luckily, moving to LLVM 8 does not seem to break too many things in PhASAR. We fixed all breaking changes in the llvm-8.0.0 branch on Github (https://github.com/secure-software-engineering/phasar/tree/llvm-8.0.0). One of the changes made to LLVM 8 worth mentioning is the removal of the terminator instruction class llvm::TerminatorInst. Instead of using llvm::dyn_cast or llvm::isa, one has to use bool llvm::Instruction::isTerminator() const in order to check for terminator instructions. After being able to successfully compile and link PhASAR against LLVM 8, we are now checking the semantically correctness of the analysis framework in combination with LLVM 8. The llvm-8.0.0 branch will be promptly merged into development after having assessed the semantically correctness.