Fix warning underlining in global init checker (#18668)

Fixes the placement of the caret when the global init checker reports
warnings for multi-line statements.

compiler/src/dotty/tools/dotc/transform/init/Trace.scala

@@ -77,7 +77,7 @@ object Trace:
    *  pos.source must exist
    */
   private def positionMarker(pos: SourcePosition): String =
-    val trimmed = pos.lineContent.takeWhile(c => c.isWhitespace).length
+    val trimmed = pos.source.lineContent(pos.start).takeWhile(c => c.isWhitespace).length
     val padding = pos.startColumnPadding.substring(trimmed).nn + "   "
     val carets =
       if (pos.startLine == pos.endLine)

tests/init-global/neg/line-spacing.check

@@ -0,0 +1,13 @@
+-- Error: tests/init-global/neg/line-spacing.scala:4:7 -----------------------------------------------------------------
+3 |    B
+4 |      .s.length // error
+  |    ^
+  |    Access uninitialized field value s. Call trace: 
+  |    -> object B {	[ line-spacing.scala:7 ]
+  |       ^
+  |    -> val s: String = s"${A.a}a"	[ line-spacing.scala:8 ]
+  |                           ^^^
+  |    -> def a: Int =	[ line-spacing.scala:2 ]
+  |       ^
+  |    -> .s.length // error	[ line-spacing.scala:4 ]
+  |       ^

tests/init-global/neg/line-spacing.scala

@@ -0,0 +1,9 @@
+object A {
+  def a: Int =
+    B
+      .s.length // error
+}
+
+object B {
+  val s: String = s"${A.a}a"
+}