Rollup merge of #122922 - kpreid:print-async, r=compiler-errors

-Zprint-type-sizes: print the types of awaitees and unnamed coroutine locals.

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.

This change will also synergize with my other PR #122923 which changes type printing to print the path of the `async fn` instead of the span.

Implementation note: I'm not sure if `Symbol::intern` is appropriate for this application, but it was the obvious way to not have to remove the `Copy` implementation from `FieldInfo`, or add a `'tcx` lifetime, while avoiding keeping a lot of possibly redundant strings in memory. I don't know what the proper tradeoff to make here is (though presumably it is not too important for a `-Z` debugging option).

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