Clang: Clean up CompilerOptionsBuilder

No behavior change.

* Remove virtual from methods that are not overridden
* Move constant member functions that do no access any members into
  source file as static functions
* Remove QLatin1String where possible
* Make variable names a bit more consistent
* Other minor stuff

Change-Id: I34a582d5a468489e11365507b283e9aee157664f
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

tests/unit/unittest/refactoringengine-test.cpp

@@ -150,9 +150,8 @@ void RefactoringEngine::SetUp()
     projectPart->files.push_back(projectFile);
 
     CompilerOptionsBuilder optionsBuilder(*projectPart);
-    commandLine = Utils::SmallStringVector(optionsBuilder.build(
-                                               projectFile.kind,
-                                               CompilerOptionsBuilder::PchUsage::None));
+    commandLine = Utils::SmallStringVector(
+        optionsBuilder.build(projectFile.kind, CppTools::UsePrecompiledHeaders::No));
     commandLine.push_back(qStringFilePath);
 }