Rollup merge of #102538 - cjgillot:def-span-ctxt, r=fee1-dead

Give `def_span` the same SyntaxContext as `span_with_body`.

rust-lang/rust#102217

I'm not sure how to add a test, since the erroneous span was crafted using a proc macro.
The debug assertion in `def_span` will ensure we have the correct behaviour.

Author: Dylan-DPC