Fix: mark relevant spans (ex: ident.span) with macro expansion context

[Quantumplation]

In #65830 it was discovered that hir::Item.ident.span isn't marked with_ctxt of being part of a macro expansion. This had two effects that we saw there:

• error messages don't include the "in this macro invocation" hints
• cases where the dead_code error was suppressed because of foreign macros were no longer suppressed

To solve that there, we're going to just use the old behavior when we're in the middle of a macro expansion already, so we don't use the un-marked span. As @estebank suggested, we should probably clean this up in a separate change.

[Quantumplation]

@estebank when you get a chance could you (or someone else) provide some light mentorship here?
Specifically,

• Where does item.span get constructed (so I can look at how the macro context gets attached)
• Where does item.ident.span get filled in?
• What other relevant spans should inherit this context?
• Are there any gotchas I should consider?