Extract QueryVisitor from HierarchicalTreeVisitor

Summary:
This change makes HierarchicalTreeVisitor visit only Cypher related AST
nodes. QueryVisitor can be used to differentiate between various query
types we have. The next step is to either rename HierarchicalTreeVisitor
to something like CypherQueryVisitor, or perhaps extract Clause visiting
from it.

Reviewers: mtomic, llugovic

Reviewed By: llugovic

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1710

tests/unit/query_variable_start_planner.cpp

@@ -32,13 +32,6 @@ std::ostream &operator<<(std::ostream &stream,
 
 namespace {
 
-auto MakeSymbolTable(query::Query &query) {
-  query::SymbolTable symbol_table;
-  query::SymbolGenerator symbol_generator(symbol_table);
-  query.Accept(symbol_generator);
-  return symbol_table;
-}
-
 void AssertRows(const std::vector<std::vector<TypedValue>> &datum,
                 std::vector<std::vector<TypedValue>> expected) {
   auto row_equal = [](const auto &row1, const auto &row2) {
@@ -67,7 +60,7 @@ void CheckPlansProduce(
     size_t expected_plan_count, query::CypherQuery *query, AstStorage &storage,
     database::GraphDbAccessor &dba,
     std::function<void(const std::vector<std::vector<TypedValue>> &)> check) {
-  auto symbol_table = MakeSymbolTable(*query);
+  auto symbol_table = query::MakeSymbolTable(query);
   auto planning_context =
       MakePlanningContext(storage, symbol_table, query, dba);
   auto query_parts = CollectQueryParts(symbol_table, storage, query);