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
6ea3ef7ac2