[SourceKit] Add decl.var.parameters entries to the fully annotated decl

This is the first part of adding parameter substructure so that clients
can reason about more of the function declaration.

rdar://problem/24292226

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -92,6 +92,8 @@ public:
   }
 
   void printDeclPre(const Decl *D) override {
+    if (isa<ParamDecl>(D))
+      return; // Parameters are handled specially in addParameters().
     unsigned StartOffset = OS.tell();
     EntitiesStack.emplace_back(D, StartOffset);
   }
@@ -104,6 +106,9 @@ public:
   }
 
   void printDeclPost(const Decl *D) override {
+    if (isa<ParamDecl>(D))
+      return; // Parameters are handled specially in addParameters().
+
     assert(EntitiesStack.back().Dcl == D);
     TextEntity Entity = std::move(EntitiesStack.back());
     EntitiesStack.pop_back();