Calculate span info on-demand

[jyn514]

• Add helper attr_span for common reused function
• Stop storing Spans on Item directly; calculate them on demand instead
• Special case modules, which have different spans depending on whether
  you use inner or outer attributes
• Special case impls with fake IDs, which can have either dummy spans (for auto traits) or the DefId of the impl block (for blanket impls)
• Use a fake ID for primitives instead of the ID of the crate; this lets
  source() know that it should use a dummy span instead of the span of
  the crate.

This shrinks Item from 48 to 40 bytes.

Helps with #76382.