Auto merge of #52536 - alexcrichton:attr-spans, r=nikomatsakis

proc_macro: Preserve spans of attributes on functions

This commit updates the tokenization of items which are subsequently passed to
`proc_macro` to ensure that span information is preserved on attributes as much
as possible. Previously this area of the code suffered from #43081 where we
haven't actually implemented converting an attribute to to a token tree yet, but
a local fix was possible here.

Closes #47941