[mypyc] Free generator after await encounters StopIteration

Previously the awaited generator could stay alive until the generator
that performed the await was freed, delaying object reclamation. The
refcount analysis doesn't understand registers spilled to the
environment, so we need to manually clear the value.

Consider code like this:
```
async def foo() -> None:
    await bar()
    await zar()
```
Previously, the `bar()` generator was only freed at end of `foo()`. Now
we release it before `await zar()`, as expected.

mypyc/irbuild/statement.py

@@ -940,6 +940,10 @@ def emit_yield_from_or_await(
     # If it wasn't, this reraises the exception.
     builder.activate_block(stop_block)
     builder.assign(result, builder.call_c(check_stop_op, [], line), line)
+    # Clear the spilled iterator/coroutine so that it will be freed.
+    # Otherwise, the freeing of the spilled register would likely be delayed.
+    err = builder.add(LoadErrorValue(object_rprimitive))
+    builder.assign(iter_reg, err, line)
     builder.goto(done_block)
 
     builder.activate_block(main_block)