LLVM is moving to a model where all pointer types in IR are opaque (https://llvm.org/docs/OpaquePointers.html) this is currently enabled-by-default for LLVM 16 and will be required for LLVM 17.

This change moves cbe to support and use opaque pointers:

• Where we have an instruction that has a helper to get the type of an operand (e.g., Value::getValueType) use that helper instead of getType()->getNonOpaquePointerElementType().
• Always emit void* for pointers
• Use a new type FunctionInfoVariant (which is a variant of Function and CallInst) instead of FunctionType for generating function declarations and prototypes, as this maintains information about struct return types, by-val param types, etc.
• Disable using opaque pointers in the test.
• When checking for an "address exposed value" also get its real type so that we can cast it correctly.
• Remove TopoligicalSorter: function declarations will not use void* for function pointers, thus it doesn't matter the order that we emit them in.
• isStandardMain can only check that the char ** parameter corresponds to a pointer parameter, rather than an actual char **.
• printGEPExpression and writeMemoryAccess will cast the pointer operand to the appropriate type.
• Cleanup printGEPExpression to remove "last index is vector" workaround.

Fixes #154