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

2 files changed, 33 insertions, 0 deletions

diff --git a/test/scaladoc/run/SI-8479.check b/test/scaladoc/run/SI-8479.check
new file mode 100644
index 0000000000..619c56180b
--- /dev/null
+++ b/test/scaladoc/run/SI-8479.check
@@ -0,0 +1 @@
+Done.
diff --git a/test/scaladoc/run/SI-8479.scala b/test/scaladoc/run/SI-8479.scala
new file mode 100755
index 0000000000..3c91395025
--- /dev/null
+++ b/test/scaladoc/run/SI-8479.scala
@@ -0,0 +1,32 @@
+import scala.tools.nsc.doc.model._
+import scala.tools.nsc.doc.base._
+import scala.tools.nsc.doc.base.comment._
+import scala.tools.partest.ScaladocModelTest
+import java.net.{URI, URL}
+import java.io.File
+
+object Test extends ScaladocModelTest {
+
+  override def code =
+    """
+      |object Test {
+      |  val x = new SparkContext(master = "")
+      |}
+      |
+      |class SparkContext(config: Any) {
+      |
+      |  /** Scaladoc comment */
+      |  def this(
+      |      master: String,
+      |      appName: String = "") = this(null)
+      |}
+      |
+      |
+    """.stripMargin
+
+  override def scaladocSettings = ""
+
+  def testModel(rootPackage: Package) {
+    // it didn't crash
+  }
+}