refactor: don't forward -1 as the diagnosticCode

This change ensures that if the diagnostic code is set to -1 that we
instead forward None as the code. This will hopefully help any tooling
consuming this to not have to add checks for -1 and filter that code
out, since it's a useless code.

Author: ckipp01

compiler/src/dotty/tools/dotc/reporting/DiagnosticRelatedInformation.scala

@@ -0,0 +1,5 @@
+package dotty.tools.dotc.reporting
+
+import dotty.tools.dotc.util.SourcePosition
+
+final case class DiagnosticRelatedInformation(msg: String, pos: SourcePosition)