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/plugins/clangcodemodel/clangmodelmanagersupport.cpp

@@ -64,12 +64,6 @@ using namespace ClangCodeModel::Internal;
 
 static ModelManagerSupportClang *m_instance = 0;
 
-static bool useClangFollowSymbol()
-{
-    static bool use = qEnvironmentVariableIntValue("QTC_CLANG_FOLLOW_SYMBOL");
-    return use;
-}
-
 static CppTools::CppModelManager *cppModelManager()
 {
     return CppTools::CppModelManager::instance();
@@ -77,16 +71,12 @@ static CppTools::CppModelManager *cppModelManager()
 
 ModelManagerSupportClang::ModelManagerSupportClang()
     : m_completionAssistProvider(m_communicator)
+    , m_followSymbol(new ClangFollowSymbol)
     , m_refactoringEngine(new RefactoringEngine)
 {
     QTC_CHECK(!m_instance);
     m_instance = this;
 
-    if (useClangFollowSymbol())
-        m_followSymbol.reset(new ClangFollowSymbol);
-    else
-        m_followSymbol.reset(new CppTools::FollowSymbolUnderCursor);
-
     CppTools::CppModelManager::instance()->setCurrentDocumentFilter(
                 std::make_unique<ClangCurrentDocumentFilter>());