Add the ability to merge spans to codemap
This PR adds the ability to merge Spans. To do so, it builds on the Codemap's ability to verify the locations of spans, namely that following can be verified:
* the expn_id of both spans much match
* the lhs span needs to end on the same line the rhs span begins
* the lhs span must start at or before the rhs span
If all of these are met, a new span is returned that is min(lo), max(hi) of the two spans.
This PR also removes an older Span merge, as this new functionality subsumes it.
r? @nrc
Revert "Remove unused methods from MultiSpan"
This reverts commit f7019a4e2f.
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
This reverts commit f7019a4e2f.
This removed the only way to make a suggestion with more than one
substitute. Bring it back until we come up with a better solution.
