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, 1 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Namers.scala b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
index bb938074cb..45da6d80d6 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Namers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
@@ -663,10 +663,7 @@ trait Namers extends MethodSynthesis {
         val m = ensureCompanionObject(tree, caseModuleDef)
         m.moduleClass.updateAttachment(new ClassForCaseCompanionAttachment(tree))
       }
-      val hasDefault = impl.body exists {
-        case DefDef(_, nme.CONSTRUCTOR, _, vparamss, _, _)  => mexists(vparamss)(_.mods.hasDefault)
-        case _                                              => false
-      }
+      val hasDefault = impl.body exists treeInfo.isConstructorWithDefault
       if (hasDefault) {
         val m = ensureCompanionObject(tree)
         m.updateAttachment(new ConstructorDefaultsAttachment(tree, null))