Better handle non-refutable patterns in async transform

Keep track of patterns that bind multiple vars and
print them out when converting an async call. If
the parameter being bound isn't referenced elsewhere,
we'll print the pattern inline as e.g:

```
let ((x, y), z) = await foo()
```

Otherwise, if the parameter is referenced elsewhere
in the block we'll print the pattern out of line,
such as:

```
let (res, z) = await foo()
let (x, y) = res
```

In addition, make sure to print var bindings out
of line if there's also a let binding, e.g:

```
let x = await foo()
var y = x
```

This ensures any mutations to y doesn't affect x.
If there's only a single var binding, we'll print
it inline.

rdar://77802560

Author: hamishknight