Clang: Use current TranslationUnit follow symbol based on clang code model

It might be quite a safe replacement which can fix builtin
code model issues.

If clang code model fails to follow symbol or does not find
a definition when it's required we fall back to the built-in
code model to proceed with project-wide follow symbol.

To make it almost a full replacement tweak include paths underline
on cursor hover to match what we have in built-in code model.

SIGNAL/SLOTS macros are not yet supported but can be handled
in follow up patch.

Task-number: QTCREATORBUG-19477
Change-Id: Id1611511d661a8aaf3e93502b4e03e1792c7c1d3
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/tools/clangbackend/source/clangcodemodelserver.cpp

@@ -261,6 +261,7 @@ void ClangCodeModelServer::requestDocumentAnnotations(const RequestDocumentAnnot
 
         DocumentProcessor processor = documentProcessors().processor(document);
         processor.addJob(JobRequest::Type::RequestDocumentAnnotations);
+        processor.addJob(JobRequest::Type::UpdateExtraDocumentAnnotations);
         processor.process();
     }  catch (const std::exception &exception) {
         qWarning() << "Error in ClangCodeModelServer::requestDocumentAnnotations:" << exception.what();