Clang: Optimize clang overview model

Build the tree in only one loop.

clangbackend generates tokens almost as fast as it
did before (about 10% slower in general).
Broken documents are more affected and take much more
time (about 300%) but it's better to have this time spent
on backend side then in QtC itself.

Task-number: QTCREATORBUG-20205
Change-Id: I34c58bca30c4494005a029abd82c7e612ecd6fb9
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

tests/unit/unittest/tokenprocessor-test.cpp

@@ -1354,6 +1354,14 @@ TEST_F(TokenProcessor, AnonymousStruct)
     ASSERT_THAT(container.types.mixinHighlightingTypes[0], ClangBackEnd::HighlightingType::Struct);
 }
 
+TEST_F(TokenProcessor, LexicalParentIndex)
+{
+    const auto containers = translationUnit.fullTokenInfosInRange(
+                translationUnit.sourceRange(50, 1, 53, 3)).toTokenInfoContainers();
+
+    ASSERT_THAT(containers[3].extraInfo.lexicalParentIndex, 1);
+}
+
 Data *TokenProcessor::d;
 
 void TokenProcessor::SetUpTestCase()