ClangCodeModel: Add experimental clangd support

If the user has enabled clangd (default is off), we start up one instance
per project when it is opened/changed (including build config switches),
and trigger background indexing.
So far, the index is used to provide results for locators and "Find
Usages".
Per-document functionality such as semantic highlighting and completion
is still provided by libclang.

Change-Id: I12532fca1b9c6278baab560e7238cba6189cde9f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangcodemodelplugin.cpp

@@ -77,7 +77,8 @@ void ClangCodeModelPlugin::generateCompilationDB()
 
     QFuture<GenerateCompilationDbResult> task
             = QtConcurrent::run(&Internal::generateCompilationDB,
-                                CppModelManager::instance()->projectInfo(target->project()));
+                                CppModelManager::instance()->projectInfo(target->project()),
+                                CompilationDbPurpose::Project);
     Core::ProgressManager::addTask(task, tr("Generating Compilation DB"), "generate compilation db");
     m_generatorWatcher.setFuture(task);
 }