fix: scoverage statement's line number should be 1-base

fix #18916

This PR makes scoverage statements' line number 1-base,
instead of 0-base, as @tarao described.

It would be ideal if we could find specifications of many of
coverage report fomats (such as Jacoco XML, cobertura.xml, lcov.txt...)
and confirm most of them expect the line numbers 1-base.

However, it seems there's no formal specification for them AFAIK.
Since we've been using 1-base so far in Scala2 and it hasn't been a problem,
so I guess it's OK to change to 1-base (and most of coverage report
format and visualizer expect 1-base maybe).

I believe it should be reporter / aggregator's responsibility to
adjust the line numbers if some coverage format expects 0-base.

Author: tanishiking

compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala

@@ -97,7 +97,9 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
         id = id,
         start = pos.start,
         end = pos.end,
-        line = pos.line,
+        // +1 to account for the line number starting at 1
+        // the internal line number is 0-base https://github.com/lampepfl/dotty/blob/18ada516a85532524a39a962b2ddecb243c65376/compiler/src/dotty/tools/dotc/util/SourceFile.scala#L173-L176
+        line = pos.line + 1,
         desc = sourceFile.content.slice(pos.start, pos.end).mkString,
         symbolName = tree.symbol.name.toSimpleName.toString,
         treeName = tree.getClass.getSimpleName.nn,