Handle the LLVM exception system in Why3 generation

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LLVM has instructions made for exception handling, which are used by compilers such as clang++. However, we currently are not able to model these instructions in WhyR. To support all of LLVM, exceptions will have to be modeled.

This is difficult for various reasons:

• It requires non-local branching in some cases
• Exception handling is scoped dynamically
• The inputs to catch blocks are decided by "personality functions" in LLVM, which are essentially black boxes