Detect if item.span is in a macro, and fall back

If item.span is part of a macro invocation, this has several downstream
implications.  To name two that were found while working on this:

 - The dead-code error gets annotated with a "in this macro invocation"
 - Some errors get canceled if they refer to remote crates

Ideally, we should annotate item.ident.span with the same macro info,
but this is a larger change (see: #66095), so for now we just fall
back to the old behavior if this item was generated by a macro.

I use span.macro_backtrace().len() to detect if it's part of a macro,
because that (among other things) is what is used by the code which
adds the "in this macro invocation" annotations mentioned above.

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