Update for the latest KLEE

[Anton Kochkov]

There were a lot of improvements and bugfixes in KLEE since:

• 2.0.0 release https://github.com/klee/klee/releases/tag/v2.0
• Newer changes: klee/klee@v2.0...master

[Sergey Mechtaev]

Thanks, we plan to update it. Currently, Angelix uses old llvm with llvm-gcc. To update KLEE, we also have to switch to newer llvm and use wllvm instead.