Add person readable toString for PositionBridge

There are compile call paths relying on
meaningful toString impl. for error/warning
log messages. Therefor lets add toString, which is:
"concise but informative representation that is easy for a person to read."

Without this, e.g. users of https://github.com/davidB/scala-maven-plugin
get unsuitable compile problems like:
[Warn] dotty.tools.xsbt.PositionBridge@4fbdc20: match may not be exhaustive....

Example call path:
log:45, SbtLogger (sbt_inc)
log:47, Logger (sbt.util)
warn:43, Logger (sbt.util)
logWarning:159, LoggedReporter (sbt.internal.inc)
display:168, LoggedReporter (sbt.internal.inc)
log:142, LoggedReporter (sbt.internal.inc)
doReport:43, DelegatingReporter (dotty.tools.xsbt)
report:150, Reporter (dotty.tools.dotc.reporting)
issueWarning:32, report$ (dotty.tools.dotc)
warning:67, report$ (dotty.tools.dotc)
checkExhaustivity:847, SpaceEngine (dotty.tools.dotc.transform.patmat)
....

Author: Markus Sutter

sbt-bridge/src/dotty/tools/xsbt/PositionBridge.java

@@ -39,6 +39,10 @@ public Optional<Integer> pointer() {
     public Optional<String> pointerSpace() {
       return Optional.empty();
     }
+
+    public String toString() {
+      return "";
+    }
   };
 
   public PositionBridge(SourcePosition pos, SourceFile src) {
@@ -116,4 +120,9 @@ public Optional<String> pointerSpace() {
       result.append(lineContent.charAt(i) == '\t' ? '\t' : ' ');
     return Optional.of(result.toString());
   }
+
+  @Override
+  public String toString() {
+      return pos.toString();
+  }
 }