Rollup merge of #35094 - mcarton:multispan, r=jonathandturner

Rollup merge of #35094 - mcarton:multispan, r=jonathandturner

Revert "Remove unused methods from MultiSpan"

This reverts commit f7019a4e2f80577d38ec35fcebd64d5970b15f78.

That commit removed the only way to make a suggestion with more than one substitute. That feature is not used directly by rustc but exists and is used by Clippy. Bring it back until we come up with a better solution (suggestions don't use span labels, so it would make sense for them to use their own type).
Rational there: https://github.com/Manishearth/rust-clippy/pull/1119.

r? @jonathandturner
Cc @Manishearth