Support added for exceptions with ASYNCIFY

*See matching emscripten pull request [#2772](https://github.com/kripken/emscripten/pull/2772).*

We add support for exceptions with asynchronous functions. Changes:

1. The LowerEmExceptions pass turns "invoke" instructions into "call" instructions plus some gloop. Rather than trying to recognise the gloop, I've added a function attribute "emscripten_invoke" to the resulting call instruction so that LowerEmAsyncify can easily spot which call instructions are inside try blocks.
2. LowerEmAsyncify mangles the invoke sequence by splitting the block right after the call instruction. I've fixed this up by adding a call to "emscripten_async_postinvoke1" just before the split on the other side of the split "emscripten_async_postinvoke2". These functions do nothing, they just set and assert the `InvokeState` variable so that illegal reordering of the invoke sequence by optimisations will be detected.
3. Finally, we've added a third instruction to `emscripten_alloc_async_context` so that a flag is set in the context indicating where it's a checked asynchronous call (invoke) or unchecked.
Good spot, I probably shouldn't have comitted that. It's an unused variable left by @coolwanglu, I think. I commented it out to make it clearer to myself that it wasn't doing anything, but didn't delete it because perhaps it was part of coolwanglu's grand plan...