Allow bench task to pause between runs

If -Xprompt is set, pause a dotty.tools.dotc.Bench task between compiler
runs. This is useful for, e.g. taking a memory snapshot at a predictable time.

compiler/src/dotty/tools/dotc/Bench.scala

@@ -1,7 +1,3 @@
-/* NSC -- new Scala compiler
- * Copyright 2005-2013 LAMP/EPFL
- * @author  Martin Odersky
- */
 package dotty.tools
 package dotc
 
@@ -24,6 +20,11 @@ object Bench extends Driver {
       val start = System.nanoTime()
       val r = super.doCompile(compiler, fileNames)
       println(s"time elapsed: ${(System.nanoTime - start) / 1000000}ms")
+      if (ctx.settings.prompt.value) {
+        print("hit <return> to continue >")
+        System.in.read()
+        println()
+      }
       r
     }