Not-awaitable diagnostic context skips some steps

[carljm]

For this code:

class NotAwaitable:
    __await__ = 1
    
async def _(na: NotAwaitable):
    await na

We emit this diagnostic:

error[invalid-await]: `NotAwaitable` is not awaitable
   --> main.py:5:11
    |
  4 | async def _(na: NotAwaitable):
  5 |     await na
    |           ^^
    |
   ::: stdlib/builtins.pyi:348:7
    |
347 | @disjoint_base
348 | class int:
    |       --- attribute defined here
349 |     """int([x]) -> integer
350 |     int(x, base=10) -> integer
    |
info: `__await__` is not callable

It's kind of strange/confusing that we jump straight to highlighting int in stdlib, when the more relevant thing to highlight would be __await__ = 1.