Forward reference error includes line numbers

[som-snytt]

Forward port of scala/scala#9654

Fixes #14401 where the line number helps the user realize they are looking at the wrong definition.

It would be nice if diagnostics could have supplementary positions for red squiggles.

This commit does not address shadowing per se.

[som-snytt]

It would be nice if supplemental squiggles were generated for an IDE but longer text with line info (for example) only in standard error output. A Diagnostic could consist of multiple parts and a mechanism to combine them into a single message (and position).

How many diagnostics would benefit from such non-local supplementary info?

This occurred to me because I considered that I don't need to be told the location of a symbol if it's on "the next line"; but that assumes I'm looking at the source (and not error output). A comment on the Scala 2 PR was that "full location string isn't useful without a caret".

scala/scala#9654 (comment)