IDE: Fix bug in SourceEntityWalker subscript handling

We weren't passing the lvalue access kind to the subscript.
This was causing a test to fail with the next patch.

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -1041,10 +1041,11 @@ public:
   }
 
   bool visitSubscriptReference(ValueDecl *D, CharSourceRange Range,
+                               Optional<AccessKind> AccKind,
                                bool IsOpenBracket) override {
     // Treat both open and close brackets equally
     return visitDeclReference(D, Range, nullptr, nullptr, Type(),
-                      ReferenceMetaData(SemaReferenceKind::SubscriptRef, None));
+                      ReferenceMetaData(SemaReferenceKind::SubscriptRef, AccKind));
   }
 
   bool isLocal(Decl *D) const {