[WebAssembly] Error when wasm EH is used with Emscripten EH/SjLj

- Error out when both Emscripten EH and wasm EH are used together, i.e.,
  both `-enable-emscripten-cxx-exceptions` and `-exception-model=wasm`
  are given together. This will not happen if you use Emscripten, but
  this can happen when you call `llc` manually with wrong set of
  arguments.
- Currently we don't yet support using wasm EH with Emscripten SjLj.
  Unlike `-enable-emscripten-cxx-exceptions` which is turned on only
  when you use `emcc -s DISABLE_EXCEPTION_CATCHING=0`,
  `-enable-emscripten-sjlj` is turned on by Emscripten by default. So we
  error out only when it is turned on and `setjmp` or `longjmp` is
  actually used.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D101403