[mypyc] Free coroutine after await encounters StopIteration (#19231)

Previously the awaited coroutine could stay alive until the coroutine
that performed the await was freed, delaying object reclamation. The
reference counting 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()` coroutine was only freed at end of `foo()`. Now
we release it before `await zar()`, as expected.

Author: JukkaL

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)