[index] Fix dynamicMemberLookup subscript reference implicit role

When building the implicit subscript expression, set the "implicit" bit
correctly and pass it through in the indexer so that we get implicit
refernces to the subscript. This would be useful for e.g. searching for
all uses of the dynamic subscript.

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -1038,11 +1038,10 @@ public:
   }
 
   bool visitSubscriptReference(ValueDecl *D, CharSourceRange Range,
-                               Optional<AccessKind> AccKind,
+                               ReferenceMetaData Data,
                                bool IsOpenBracket) override {
     // Treat both open and close brackets equally
-    return visitDeclReference(D, Range, nullptr, nullptr, Type(),
-                      ReferenceMetaData(SemaReferenceKind::SubscriptRef, AccKind));
+    return visitDeclReference(D, Range, nullptr, nullptr, Type(), Data);
   }
 
   bool isLocal(Decl *D) const {