Handle dynamic member lookup in annotation and cursor info

Ensure the various entity walkers handle the implicit subscript
reference correctly (usually by ignoring it) and fall through to the
underlying declarations.

rdar://49028895

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -1032,6 +1032,8 @@ public:
   bool visitDeclReference(ValueDecl *D, CharSourceRange Range,
                           TypeDecl *CtorTyRef, ExtensionDecl *ExtTyRef, Type Ty,
                           ReferenceMetaData Data) override {
+    if (Data.isImplicit)
+      return true;
     unsigned StartOffset = getOffset(Range.getStart());
     References.emplace_back(D, StartOffset, Range.getByteLength(), Ty);
     return true;