[CodeCompletion] Don't attach attr to decl if blank line exists

```
@#^COMPLETE^#

public func something() {}
```
In this case, we can't say the user is adding attribute to the func or
starting a new declaration. So if there're one or more blank lines after the
completion, suggest context free attribute list.

rdar://problem/50441643

include/swift/Parse/CodeCompletionCallbacks.h

@@ -189,7 +189,7 @@ public:
   virtual void completeAccessorBeginning(CodeCompletionExpr *E) {};
 
   /// Complete the keyword in attribute, for instance, @available.
-  virtual void completeDeclAttrBeginning(bool Sil) {};
+  virtual void completeDeclAttrBeginning(bool Sil, bool isIndependent) {};
 
   /// Complete the parameters in attribute, for instance, version specifier for
   /// @available.