This makes `-Zprint-type-sizes`'s output easier to read, because the
name of an `async fn` is more immediately recognizable than its span.
I also deleted the comment "FIXME(eddyb) should use `def_span`." because
it appears to have already been fixed by commit 67727aa7c3.
This should assist comprehending the size of coroutines.
In particular, whenever a future is suspended while awaiting another
future, the latter is given the special name `__awaitee`, and now the
type of the awaited future will be printed, allowing identifying
caller/callee — er, I mean, poller/pollee — relationships.
It would be possible to include the type name in more cases, but I
thought that that might be overly verbose (`print-type-sizes` is already
a lot of text) and ordinary named fields or variables are easier for
readers to discover the types of.
Wrap coroutine variant fields in MaybeUninit to indicate that they
might be uninitialized. Otherwise an uninhabited field will make
the entire variant uninhabited and introduce undefined behaviour.
The analogous issue in the prefix of coroutine layout was addressed by
6fae7f8071.
This adds one more test that should track improvements to generator
layout, like #62958 and #62575.
In particular, this test highlights suboptimal layout, as the storage
for the argument future is not being reused across its usage as `upvar`,
`local` and `awaitee` (being polled to completion).
