The `FnAbi` now knows if the function is allowed to unwind. If a
function isn't allowed to unwind, we can use a `call` instead of an
`invoke`.
This resolves an issue when calling LLVM intrinsics which cannot unwind
LLVM will generate an error if you attempt to invoke them so we need to
ignore cleanup blocks in codegen and generate a call instead.
8da26e0366