ClangCodeModel: Speed up compilation db generation

We needlessly re-evaluated the same compiler options again and again for
all files in a project part.
Now we only do the actual file-related work per file. Along the way, we
dissolved some unneeded classes and made CompilerOptionsBuilder non-
polymorphic.

Change-Id: I9710d641a57032936cc0812515974dbc91676c8c
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangcodemodelplugin.cpp

@@ -80,11 +80,13 @@ void ClangCodeModelPlugin::generateCompilationDB()
     if (!projectInfo)
         return;
 
+    const CppEditor::ClangDiagnosticConfig warningsConfig
+            = warningsConfigForProject(target->project());
     QFuture<GenerateCompilationDbResult> task
             = QtConcurrent::run(&Internal::generateCompilationDB, projectInfo,
                                 projectInfo->buildRoot(), CompilationDbPurpose::Project,
-                                warningsConfigForProject(target->project()),
-                                optionsForProject(target->project()));
+                                warningsConfig,
+                                optionsForProject(target->project(), warningsConfig));
     Core::ProgressManager::addTask(task, tr("Generating Compilation DB"), "generate compilation db");
     m_generatorWatcher.setFuture(task);
 }