Do not use the plain printer with -Ydebug

Instead, a new setting called -Yplain-printer is used for this.
After this commit, we can now run all tests with -Ydebug (this was not
the case before because using the plain printer breaks -Ytest-pickler)

Author: smarter

src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -150,6 +150,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val Yheartbeat = BooleanSetting("-Yheartbeat", "show heartbeat stack trace of compiler operations.")
   val Yprintpos = BooleanSetting("-Yprintpos", "show tree positions.")
   val YnoDeepSubtypes = BooleanSetting("-Yno-deep-subtypes", "throw an exception on deep subtyping call stacks.")
+  val YplainPrinter = BooleanSetting("-Yplain-printer", "Pretty-print using a plain printer.")
   val YprintSyms = BooleanSetting("-Yprint-syms", "when printing trees print info in symbols instead of corresponding info in trees.")
   val YtestPickler = BooleanSetting("-Ytest-pickler", "self-test for pickling functionality; should be used with -Ystop-after:pickler")
   val YcheckReentrant = BooleanSetting("-Ycheck-reentrant", "check that compiled program does not contain vars that can be accessed from a global root.")

src/dotty/tools/dotc/printing/Printers.scala

@@ -8,7 +8,7 @@ trait Printers { this: Context =>
   /** A function creating a printer */
   def printer = {
     val pr = printerFn(this)
-    if (this.debug) pr.plain else pr
+    if (this.YplainPrinter) pr.plain else pr
   }
 }