Hide stack traces behind -Ydebug

They're not very useful for end users and some tests like
tests/neg/selfreq.scala always print these exceptions which makes it
harder to read the test logs,

Also use Thread.dumpStack() instead of creating an Exception and calling
printStackTrace() on it.

Author: smarter

src/dotty/tools/dotc/core/Types.scala

@@ -3386,7 +3386,7 @@ object Types {
   class MissingType(pre: Type, name: Name)(implicit ctx: Context) extends TypeError(
     i"""cannot resolve reference to type $pre.$name
        |the classfile defining the type might be missing from the classpath${otherReason(pre)}""".stripMargin) {
-    printStackTrace()
+    if (ctx.debug) printStackTrace()
   }
 
   private def otherReason(pre: Type)(implicit ctx: Context): String = pre match {

src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -186,7 +186,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
     val ex = new BadSignature(
       sm"""error reading Scala signature of $classRoot from $source:
           |error occurred at position $readIndex: $msg""")
-    /*if (debug)*/ original.getOrElse(ex).printStackTrace() // !!! DEBUG
+    if (ctx.debug) original.getOrElse(ex).printStackTrace()
     throw ex
   }
 
@@ -423,7 +423,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
               owner.info.decls.checkConsistent()
               if (slowSearch(name).exists)
                 System.err.println(i"**** slow search found: ${slowSearch(name)}")
-              new Exception().printStackTrace()
+              if (ctx.debug) Thread.dumpStack()
               ctx.newStubSymbol(owner, name, source)
             }
           }

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

@@ -32,7 +32,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
 
   protected def recursionLimitExceeded() = {
     ctx.warning("Exceeded recursion depth attempting to print.")
-    (new Throwable).printStackTrace
+    if (ctx.debug) Thread.dumpStack()
   }
 
   /** If true, tweak output so it is the same before and after pickling */

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

@@ -63,7 +63,7 @@ class ConsoleReporter(
     if (reader != null) {
       val response = reader.read().asInstanceOf[Char].toLower
       if (response == 'a' || response == 's') {
-        (new Exception).printStackTrace()
+        Thread.dumpStack()
         if (response == 'a')
           sys.exit(1)
       }

src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -327,7 +327,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
       private[TreeMakers] def incorporateOuterRebinding(outerSubst: Rebindings): Unit = {
         if (currSub ne null) {
           ctx.debuglog("BUG: incorporateOuterRebinding called more than once for " + ((this, currSub, outerSubst)))
-          Thread.dumpStack()
+          if (ctx.debug) Thread.dumpStack()
         }
         else currSub = outerSubst >> rebindings
       }