Rollup merge of #89046 - oli-obk:fix_oflo, r=estebank

"Fix" an overflow in byte position math

r? `@estebank`

help! I fixed the ICE only to brick the diagnostic.

I mean, it was wrong previously (using an already expanded macro span), but it is really bad now XD

diff --cc compiler/rustc_errors/src/lib.rs
Simple merge

diff --cc compiler/rustc_parse/src/parser/expr.rs
Simple merge

diff --cc compiler/rustc_span/src/lib.rs
Simple merge