SI-8479 Fix constructor default args under scaladoc

The `DocDef` node hid the `DefDef` constructor from the scrutinee
of the namer when determining if the class had constructor defaults
or not.

The current pattern for fixing these bugs is to delegate the check
to `TreeInfo`, and account for the wrapper `DocDef` node. I've
followed that pattern, but expressed my feelings about this approach
in a TODO comment.

Before this patch, the enclosed test failed with:

     error: not enough arguments for constructor SparkContext: (master: String, appName: String)SparkContext

1 files changed, 5 insertions, 0 deletions

diff --git a/src/reflect/scala/reflect/internal/TreeInfo.scala b/src/reflect/scala/reflect/internal/TreeInfo.scala
index fa4441e513..a6556fc22d 100644
--- a/src/reflect/scala/reflect/internal/TreeInfo.scala
+++ b/src/reflect/scala/reflect/internal/TreeInfo.scala
@@ -50,6 +50,11 @@ abstract class TreeInfo {
     case _ => false
   }
 
+  def isConstructorWithDefault(t: Tree) = t match {
+    case DefDef(_, nme.CONSTRUCTOR, _, vparamss, _, _)  => mexists(vparamss)(_.mods.hasDefault)
+    case _                                              => false
+  }
+
   /** Is tree a pure (i.e. non-side-effecting) definition?
    */
   def isPureDef(tree: Tree): Boolean = tree match {