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/clangmodelmanagersupport.cpp

@@ -419,10 +419,10 @@ void ClangModelManagerSupport::updateLanguageClient(
         });
 
     });
+    const ClangDiagnosticConfig warningsConfig = warningsConfigForProject(project);
     auto future = Utils::runAsync(&Internal::generateCompilationDB, projectInfo, jsonDbDir,
                                   CompilationDbPurpose::CodeModel,
-                                  warningsConfigForProject(project),
-                                  optionsForProject(project));
+                                  warningsConfig, optionsForProject(project, warningsConfig));
     generatorWatcher->setFuture(future);
     m_generatorSynchronizer.addFuture(future);
 }