Avoid too eager computation in reporting.

I noted a slowdown of about 25% (66sec -> 81sec) when compiling dotty even
after the subtype optimization (before it was 117sec).
I tracked it down to the traceIndented fix which avoided questions to be evaluated
twice. But it also caused the question to be evaluated
Making the val lazy fixed the problem.

Author: odersky

src/dotty/tools/dotc/reporting/Reporter.scala

@@ -155,7 +155,7 @@ trait Reporting { this: Context =>
     else {
       // Avoid evaluating question multiple time, since each evaluation
       // may cause some extra logging output.
-      val q: String = question
+      lazy val q: String = question
       traceIndented[T](s"==> $q?", (res: Any) => s"<== $q = ${resStr(res)}")(op)
     }
   }