This adds three tests for mixing exceptions and sjlj.
We currently pass `test_exceptions_longjmp1`, and other two tests will
pass after https://reviews.llvm.org/D106525 lands.

- `test_exceptions_longjmp1`
  In a function with a `setjmp` call, `invoke` exists in LLVM IR. We have to check
  both for EH and sjlj when a function throws.
- `test_exceptions_longjmp2`
  In a function without a `setjmp` call, `invoke` exists in LLVM IR. We
  only have to handle exceptions, but when longjmps occur, we shouldn't
  swallow them.
- `test_exceptions_longjmp3`
  In a function with a `setjmp` call, there's no `invoke` in LLVM IR. We
  only have to handle longjmps, but w hen an exceptions occcur, we
  shouldn't swallow them.

Each of these tests corresponds with tests in
test/CodeGen/WebAssembly/lower-em-sjlj.ll in
https://reviews.llvm.org/D106525.