Rollup merge of #122923 - kpreid:print-async-def, r=compiler-errors

In `pretty_print_type()`, print `async fn` futures' paths instead of spans.

This makes `-Zprint-type-sizes`'s output easier to read, because the name of an `async fn` is more immediately recognizable than its span. This change will also synergize with my other `-Zprint-type-sizes` PR #122922 which prints the type of child futures being awaited.

I also deleted the comment "FIXME(eddyb) should use `def_span`." because it appears to have already been fixed by commit 67727aa7c31a24ea73a91a9134c3653fae8209ab.

Author: workingjubilee